The embedded music player that is present in a number of pages on ListenBrainz can optionally be connected to play YouTube content. During the connection process ListenBrainz will request permissions to view your YouTube profile data. Unfortunately this is the minimum permission level offered by YouTube and thus if we wish to offer the ability to search for YouTube content on ListenBrainz pages, we are required to ask for this permission, even though we do not need or want access to your YouTube profile. Your data is used only to authenticate to your YouTube account and for no other purposes.
ListenBrainz does not actually look at your YouTube profile. We do not inspect, download or in any way save any data that is available in your YouTube profile. None, at all.
If you wish to inspect our source code that is running on our servers, please take a look at our implementation of how we interact with your YouTube profile. Furthermore, in utils.tsx you can inspect the function searchForYoutubeTrack which shows the single YouTube API function that ListenBrainz uses.