Read your margin agreement. If it has a re-hypothecation clause you've given them the right. Most clauses say little or nothing about compensation, but not all.
It seems like this is used mostly with the wealthy...People with millions of dollars, pension funds, mutual funds, hedge funds, wealth management services. I know brokers lend stock, but I don't know to what extent they lend to other brokers. Do I have the general concept correct?
So who are most likely to lend out their shares? Could it be as easy/common for a Robinhood client, who buys 100 shares of GameStop or AMC and then makes the loan?? That small of a client??
Has nothing to do with the client - has to do with the aggregate and if there is a custody/clearing firm involved or they run their own box. Read your agreement and you discover virtually any fully paid securities can be lent. Most size institutions use a custody agent. Even OCC has a stock loan business.