Noobie here ! Got the Code - Got the Token - But who is it?

API
2

Call the validate token endpoint

Example: Token Checker | Twitch API Example

Or call the Get Users endpoint

Example: Twitch Implicit Auth Example

or since you seem to be using OIDC (id_token) call the user info endpoint

Which as per well known

https://id.twitch.tv/oauth2/.well-known/openid-configuration

is

https://id.twitch.tv/oauth2/userinfo

example: https://github.com/BarryCarlyon/twitch_misc/blob/main/authentication/oidc_authentication/server.js#L224