Every rotation in 3D fixes an axis, as per Euler’s rotation theorem from 1775.
The composition of two rotations is a rotation, where the first rotation fixes an axis, the second rotation fixes another axis, and the composition fixes a third axis.
To represent rotations using quaternions: a rotation by θ about an axis given by a unit vector v corresponds to a specific quaternion.
Composing rotations represented by quaternions is straightforward, as rotating by two quaternions q and r is equivalent to rotating by rq.