Thanks for the question.

These are called leaf galls and are areas where adult eriophyid mites laid eggs.  The structures that you are observing contain mite larvae that hatched from these eggs.  The presence of these larvae stimulated this unusual plant growth.  No harm whatsoever is done to the tree.  They typically only cause cosmetic damage

See:

https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls

Thanks for using this forum.