After you load a glTF model, this is the machinery that plays it: an AnimationMixer drives AnimationClips through AnimationActions. Crossfade between two clips and feel the blend — the same API a character controller uses for idle→walk→run.
Clips (crossfade on switch)
These clips are built in code with KeyframeTracks — no model file. The mixer is advanced every frame with mixer.update(delta); forget that call and nothing moves.