When I implemented Oauth2 (Authorization Code Grant, both sides), I found this guide to be quite helpful: https://alexbilbie.github.io/guide-to-oauth-2-grants/

One thing I found after a while: even though the refresh tokens should theoretically not expire, many sites do expire them. You have to refresh every once in a while to maintain a usable refresh token.

Many people will tell you to "just use a library", but I found that the contact surface of oauth with your app is quite large, such that a library might not actually help much. This (among other reasons) is why I wrote my own implementation (Clojure).

FWIW, this is fairly out of date - password grant must not be used and authorization code should be used in place of implicit. I highly recommend anyone dealing with OAuth to read the BCP and not just the spec, especially if you're rolling your own:

https://datatracker.ietf.org/doc/html/rfc9700

As for your API surface, typically you'd handle this at the gateway level, then individual services don't have to perform authorization.

> the refresh tokens should theoretically not expire

That is subjective. In essence, they should last long enough so the client can use them to get new access token without the user(resource owner) having to authorise a new grant. Each client is different with different needs and the scopes might be too sensitive to provide a long lasting access. So as usual, it depends.

In my server implementation, access tokens are valid for one hour and refresh tokens for 30 days. I also return refresh tokens with each access token request, so as long as the client makes at least one request per month, they do not have to bother the user for a new grant.

I just wish the spec would have a dedicated "refresh_expires_in" field in addition to "expires_in" for refresh tokens, so the client would be better informed about this. As refresh tokens are part of the spec, though optional, their life span information is lacking here.