The autofill code is actually Safari's, not the password manager. The password manager basically just needs to impmlement ASCredentialProviderViewController (https://developer.apple.com/documentation/authenticationserv...) and include the "AutoFill Credential Provider" entitlement and safari will ask if you want to sign in with Kenpass for websites and apps.

For firefox (desktop) it's a bit harder, you have to actually implement login box detection. It works like half the time, but I don't really mind much, because it's not a huge deal to just use the clipboard. Even when using 1Password I would so often find that the password autofill was so broken (or would constantly offer me to save passwords I don't want to, or for things that aren't even login boxes) that I actually actively hated it. When creating logins, I much prefer taking an explicit step to activate my password manager to do so, and when prompting for logging in, it's not a huge deal to simply copy/paste when the simple detection doesn't work.