OAuth 2

Hey everyone, basically all what I have learn so far is doing cruds, front and back (react, expres) now I have to do authentication with OAuth 2 , and I don't have any idea how I looked into YouTube and most things are outdated and I don't have any idea how to link between the back and the front with OAuth is it like any crud thing (just make an api and call it) I am just a beginner, open for any recommendations if they are free Thanks all