[klausa]

...yes? I can't tell if you're trolling at this point or genuinely unaware.

Both iOS and Android have APIs for this, you (as the app developer) just mark the relevant fields in the app as login/password/etc, and the OS will interact with your chosen password manager to autofill and/or save them.

[koakuma-chan]

Well then I don't know whom to blame, 1Password or app developers not marking the fields correctly.

[klausa]

If you've never seen this work on your device, then you might have something configured incorrectly — many app developers are incompetent and bad at this; but not _all_ of them.