I lead product development at Google for the one-tap/auto sign-in library, we designed it such that the library includes the Credential Management API and extends to provide assistance in account creation, secure passwordless, and cross-browsers support.
In particular, if you make a request for existing credentials with code like this:
googleyolo.retrieve({
supportedAuthMethods: [
"https://accounts.google.com",
"googleyolo://id-and-password"
],
supportedIdTokenProviders: [
{ uri: "https://accounts.google.com", clientId: "CLIENT_ID" }
]
});
then any saved username/passwords from the Credential Management API will be returned (in browsers supporting the API) along with token data for Google Accounts. The one-tap/auto sign-in JavaScript library wraps the Credential Management API for credential retrieval.
Furthermore, the library provides a googleyolo.hint method to show an email selector for one-tap selection of a verified email address to assist in new account creation, or to link to an existing account, and then be auto signed-in next time with token instead of password, across all browsers, so long as the same Google Account is active.
I'd suggest using the one-tap/auto sign-in library and consuming tokens as well as passwords in order to get assisted sign-up, keep existing users signed-in automatically, and provide functionality even if the browser does not support the Credential Management API.
As for the question about using your own database of username / password, the hope with this library is you could implement the ability to create accounts and auto sign-in to these and existing accounts with an OpenID Connect ID tokens representing the user's identity. With the one-tap / auto sign-in UX, these are not only much more usable, but far more secure then passwords and mitigate creation of weak/re-used passwords. Please consider this or, even better, a hosted auth solution like Firebase Auth or Auth0 and include the one-tap UX in the frontend UI.