I appended an edit to my original response.
If you have a User Access Token you can use that for every endpoint. (As long as the key remains valid; a user can choose to deactivate a granted access token in https://www.twitch.tv/settings/connections (Make sure to catch invalid responses if a user invalidates their token) )
As for endpoints where you can’t use an Application Access Token, endpoints will list a Required scope: such as channel_subscriptions in APIv5 subscribers https://dev.twitch.tv/docs/v5/reference/channels#get-channel-subscribers These require the User Access Token with the listed scope.