It's not that complicated. You should not be implementing OAuth from scratch. I don't roll my own auth. I use things like Firebase Auth for free sites or Auth0

It's not that complicated. You should not be implementing OAuth from scratch. I don't roll my own auth. I use things like Firebase Auth for free sites or Auth0 for paid sites. Adding extra auth mechanisms is trivial using those tools. If I was rolling my own auth, I would implement email and Google using something like passport.js. It's really not that difficult.