All UMovieSceneSequencePlayer have a TickManager which is an UMovieSceneSequenceTickManager (see UMovieSceneSequencePlayer::InitializeForTick).
The ticking is done in UMovieSceneSequenceTickManager::TickSequenceActors.
It eventually calls TickFromSequenceTickManager for IMovieSceneSequenceTickManagerClient which by default is implemented as UMovieSceneSequencePlayer::TickFromSequenceTickManager.
TickableClients are added with UMovieSceneSequenceTickManager::RegisterTickClient.
Which is called from ALevelSequenceActor::SetSequence → UMovieSceneSequencePlayer::Initialize.