degrees of rotation about axis (normalized for you), right-hand rule. A zero-length axis names no rotation, so Identity comes back rather than a NaN that would silently hide the entity.
The shortest-arc rotation taking from onto to. Length is irrelevant — only the two directions are — so displacements like target - muzzle can be passed straight in.
Aims an entity along forward: the rotation that takes local −Z (the glTF/Filament "forward" axis) onto forward, keeping local +Y as close to up as it can. This is the turret / billboard / chase-camera building block.
Normalized linear interpolation — cheaper than slerp but not constant-rate. Fine for per-frame blending between nearby orientations. Takes the short way round, as slerp does.
Shortest-path spherical interpolation: t = 0 gives a, t = 1 gives b, and the path between them turns at a constant rate. The one to reach for when blending poses.