It's actually great. How it works most of the time is you highlight the text box in whatever app, and if proton thinks its a login box (it has like 90% accuracy) it will make a button pop up above the keyboard. Tap it, it opens proton and suggests the account it thinks this app uses. You can tap fill or search for another account. You can then tell it to always use this account for this app, or only this time. Then it goes back to the app you were in automatically and fills it. Next time you fill it there, it doesn't need to open the app, it will just fill it.
This requires that you give it screen reading permissions IIRC but you can disable that. If you dont want this feature. Also, if you have auto lock enabled it will ask you for your password or biometric (if enabled) before auto filling or opening automatically.
I used to use dash lane and I've found that proton works a bit better than that on my pixel 7.
Oh and if you're using a browser it will not ask "every time for this app" and will try to use the website you're connected to instead. I think.