useDiscussion() occurs in three components:
CommentCount.importable to show the number of comments at the top of an article
Discussion to load the comments section
DiscussionMeta.importable to load the number of comments and the user details on the left of the comments section (via the SignedInAs component)

The getIdapiUserData component provides user data to the contributions (email specifically) and braze components (user identifiers like brazeUuid)
Worth noting there are various urls that get sent from frontend through the config model, such as idApiUrl, discussionApiUrl, mmaUrl (which defaults to manage.theguardian.com if not provided by the config), idUrl (which defaults to profile.theguardian.com). These then get passed down to components mentioned above, such as the HeaderTopBarMyAccount.
The members-data-api only really seems to be used by frontend, though DCR does consume some of the cookies that get set by the api (e.g. hide_support_messaging_cookie)

See doc