This is a super theory covering all aspects and evidence to point out how the origins of playable Octolings differ from those of Enemy Octolings. It's going to be an extremely long one full of evidence and explanation, with tons explanations included, so without further ado, let's get started:
So first things first, reproduction, it's the necessary first step to understanding the origins and the differences. So it's already common knowledge that tentacle Octarians like the Octotrooper, Octocopter, and many more are created by cutting off a tentacle from a host, most likely the host being a playable Octoling given playable Octoling hairstyles tend to be whole tentacles of varying shapes and sizes. However, playable Octolings themselves actually have a different origin, for a playable Octoling to be born, there needs to be both a male and a female playable Octoling involved. The resulting offspring looks like a small octopus, think Smollusk as a base for what a baby playable Octoling looks like. Then the playable Octoling will go into the same developmental process as an Inkling when they grow up.
Now the enemy Octolings on the other hand, might have a different, artificial, and darker origin. First things first, Octotroopers who become more humanoid result in the enemy Octolings. This process is a lot more painful given the Octotrooper is forced to develop into a being that they are not meant to develop into. There was in fact a case where said experiment actually failed: the Octo Samurai. For those who haven't played any of Splatoon 2's story mode, the Octo Samurai is a large tentacle Octarian who is roughly more humanoid than your typical tentacle Octarian, oh and he can respawn after being splatted, however, he doesn't have an octopus form of any kind. This implies that he was the first experiment in trying to create an Octoling from a tentacle, only resulting in half of the results.
The screams inside the domes of Octo Valley and in the Octo Samurai's dome could actually belong to Octotroopers undergoing this false developmental process, and it's even more fitting that the only dome in Octo Canyon to have those screams is the Octo Samurai's dome. This could explain why Marina possibly deleted pictures of her as very young, as the Marina we know was possibly an Octotrooper who went through that same painful process, her mother is a playable Octoling who donated one of her tentacles to help the Octarian army, but she possibly wanted to donate only one. Also, Marina possibly doesn't want anyone from the surface to know that she was an Octotrooper, not even Pearl, explaining why she never brought that kind of data with her to the surface, possibly encrypting said data so it couldn't be leaked.
This explains why the enemy Octolings actually look identical, they are all from the same female playable Octoling (well, a different one than Marina's mother), who all went into the same painful, artificial process of development. This could also explain how in the Splatlands, the are no records of any tentacle Octarians living there, despite Octolings living there for generations. It's possible that in the Splatlands, an Octoling cutting off one of their tentacles is considered a taboo in the Splatlandian society, which is why the decoration based on the Hotlantis trash can that has a single tentacle poking out is called the Suspicious Garbage Can, keyword Suspicious. The enemy Octolings from the Return of the Mammalians imply that a third female playable Octoling was donating her tentacles, but she could've escaped to the surface given these are the only ones with this skin tone.
Now for the lore bits shown by Nintendo, Paul is the result of a male Octoling and a female Octoling reproducing together, as younger Paul looks like an octo version of a juvenile Inkling. Warabi is definitely confirmed to have two parents, given the more accurate translation of the Japanese version of the description includes the word "both". And finally, not every female Octoling can join the unit in which the enemy Octolings are part of, this one implies that the playable female Octolings are not allowed to join this Octoling unit, as it's reserved for Octotroopers who underwent the artificial developmental process.
Wow, this super theory ended up even longer than I expected, but hopefully, it will clear up the confusion of the Octoling developmental process.
So first things first, reproduction, it's the necessary first step to understanding the origins and the differences. So it's already common knowledge that tentacle Octarians like the Octotrooper, Octocopter, and many more are created by cutting off a tentacle from a host, most likely the host being a playable Octoling given playable Octoling hairstyles tend to be whole tentacles of varying shapes and sizes. However, playable Octolings themselves actually have a different origin, for a playable Octoling to be born, there needs to be both a male and a female playable Octoling involved. The resulting offspring looks like a small octopus, think Smollusk as a base for what a baby playable Octoling looks like. Then the playable Octoling will go into the same developmental process as an Inkling when they grow up.
Now the enemy Octolings on the other hand, might have a different, artificial, and darker origin. First things first, Octotroopers who become more humanoid result in the enemy Octolings. This process is a lot more painful given the Octotrooper is forced to develop into a being that they are not meant to develop into. There was in fact a case where said experiment actually failed: the Octo Samurai. For those who haven't played any of Splatoon 2's story mode, the Octo Samurai is a large tentacle Octarian who is roughly more humanoid than your typical tentacle Octarian, oh and he can respawn after being splatted, however, he doesn't have an octopus form of any kind. This implies that he was the first experiment in trying to create an Octoling from a tentacle, only resulting in half of the results.
The screams inside the domes of Octo Valley and in the Octo Samurai's dome could actually belong to Octotroopers undergoing this false developmental process, and it's even more fitting that the only dome in Octo Canyon to have those screams is the Octo Samurai's dome. This could explain why Marina possibly deleted pictures of her as very young, as the Marina we know was possibly an Octotrooper who went through that same painful process, her mother is a playable Octoling who donated one of her tentacles to help the Octarian army, but she possibly wanted to donate only one. Also, Marina possibly doesn't want anyone from the surface to know that she was an Octotrooper, not even Pearl, explaining why she never brought that kind of data with her to the surface, possibly encrypting said data so it couldn't be leaked.
This explains why the enemy Octolings actually look identical, they are all from the same female playable Octoling (well, a different one than Marina's mother), who all went into the same painful, artificial process of development. This could also explain how in the Splatlands, the are no records of any tentacle Octarians living there, despite Octolings living there for generations. It's possible that in the Splatlands, an Octoling cutting off one of their tentacles is considered a taboo in the Splatlandian society, which is why the decoration based on the Hotlantis trash can that has a single tentacle poking out is called the Suspicious Garbage Can, keyword Suspicious. The enemy Octolings from the Return of the Mammalians imply that a third female playable Octoling was donating her tentacles, but she could've escaped to the surface given these are the only ones with this skin tone.
Now for the lore bits shown by Nintendo, Paul is the result of a male Octoling and a female Octoling reproducing together, as younger Paul looks like an octo version of a juvenile Inkling. Warabi is definitely confirmed to have two parents, given the more accurate translation of the Japanese version of the description includes the word "both". And finally, not every female Octoling can join the unit in which the enemy Octolings are part of, this one implies that the playable female Octolings are not allowed to join this Octoling unit, as it's reserved for Octotroopers who underwent the artificial developmental process.
Wow, this super theory ended up even longer than I expected, but hopefully, it will clear up the confusion of the Octoling developmental process.