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. This calls UMovieSceneSequencePlayer::UpdateAsync → UMovieSceneSequencePlayer::Update → UMovieSceneSequencePlayer::UpdateTimeCursorPosition
TickableClients are added with UMovieSceneSequenceTickManager::RegisterTickClient.
Which is called from ALevelSequenceActor::SetSequence → UMovieSceneSequencePlayer::Initialize.