No. With Twitch OIDC both a OIDC Token and an access token are returned at the same time.
Most OIDC applications only utilise the OIDC token, the access token is an “extra” thing Twitch has.
If you OIDC with extra scopes other than the OIDC scope. The scopes are applied to the access token for calling the “regular” Twitch API with, rather than just the ODIC token on the OIDC endpoints
Depending on which OIDC flow you use will depend on what happens and what you do.
If you use “regular” OIDC auth,
the user is sent to Twitch
they accept (or decline) the link
they get redirected to your site
you get a ?code query paramter to exchange for a OIDC token (+ access token).
I don’t understand what you mean with this question.
Sure you could do this validation client side, but then the client could spoof this validation. Granted that would be unlikely to have the right data in the token in the first place
However to truly validate you’d grab the access token and call the userinfo endpoint with it anyway Getting Tokens: OIDC | Twitch Developers if using “regular” OIDC