Your question intrigued me so now I’m also looking. This is the clearest vid I’ve found. Basically, I think the wool fibres are already a bit long (3-12cm based on coarseness) and the spinning binds them in such a way that their lengths overlap and add up.
youtu.be/YwRbyTCqOQY?...
youtu.be
How Wool Is Made: From Farm to Fashion
YouTube video by Woolmark