Editor
The displayed menu for tags in tracks is FObjectBindingModel.
Inside FObjectBindingModel::AddTagMenu you can see how the tags are queried (FObjectBindingTagCache::IterateTags).
Inside FObjectBindingTagCache::ConditionalUpdate UMovieScene::AllTaggedBindings is called and iterated to fill the internal data this tag cache object maintains.
ConditionalUpdate is called from FSequencer::Tick.
The UMovieScene only holds a TMap of tags in BindingGroups.
It is only edited via the Binding functions such as AddNewBindingTag (which is not used in engine) or TagBinding.
When playing with AddNewBindingTag it doesn’t show up in the tag list since it has no binding IDs as value in the TMap pair, but it will show up on the bottom horizontal list in the Tag Binding Manager (which isn’t usable).