It's fantastic, and for some reason I trust OS/browser developers to do this more safely than a company focused on password management that has to figure out OS APIs, write browser extensions, or rely on a clipboard that has nearly unbounded read access.
Android's autofill framework is open to everyone to use, and every third-party password manager has a Chrome plugin. I use Bitwarden with exactly this experience, but across Firefox and Chrome and Android.
At least on iOS, this works for any password manager.
and where do you store your passwords for apps?
I use KeePassXC to store passwords for apps.
So you have two password managers one for Firefox and the other for apps. What happens if you have an app login that is also a web site? Two entries of the same thing?