I’ve seen two papers on stimulation and entrainment of brain waves. Is there any place that has a huge pile of resources on that? Everything from introductory to cutting-edge papers?

"neuromodulation" mostly means deep brain stimulation since it's by far the most effective and clinically - if you pubmed stuff, there's reams of reams of papers (probably written by someone else than Ali Rezai...). That being said, anything re above neurophysiology is mostly still handwaving, bc honestly, we still don't know exactly how it works.