Thanks.
Makes sense except terminology, for example: “ You sell what you just borrowed, say for $250 a share, and get a $25,000 cash loan.”
If you sold it why is it a loan? Why isn’t it just that you get $25,000 on the sale?
It's actually the broker who sold someone else's stock. That was in order to loan the "short seller" the proceeds from the sale. The "short seller" then has money he didn't have before - a loan. But it's the broker who's on the hook to replace the other guy's stock at some point, whether it's then worth more or less, so besides loaning the proceeds, the broker makes it the "short seller's" obligation to replace the stock at some point whether it costs more or less.
The broker keeps track of all this. The "short seller" has a negative number of shares of the stock he made a "bet" on recorded in his account, and also a loan of the money from those shares having been sold.
If the stock falls in value, the "short seller" at some point buys it back on the cheap, closing the loan on his own, whereupon the broker returns it to the original owner and collects any remaining interest owed on the monetary loan. The original owner never knows his stock has been written up in the margin as on-loan, that's the broker's prerogative for the pool of traders that have signed up for margin trading (meaning they can borrow from the broker).
If the stock rises in value, it may start to appear that the "short seller" won't have enough money for the broker to make the original stock owner whole. The broker then calls in the loan (the original proceeds) and also takes enough cash or other assets that the "short seller" owns to buy back the stock at it's higher price. The broker also takes enough for his just compensation (interest he hasn't already collected and any fees) and closes the loan.