I'm an attorney and accountant, not an economist or finance person. The basic explanation about "borrow, sell, buy, return" is from investopedia.
The bottom line is that stock is OWNERSHIP of PROPERTY. Just as you can LEND somebody your car and they can use it for Uber or they could RENT it to someone, if you LEND stock, THEY get to keep INCOME, etc.
Why would someone LEND stock? They might actually be RENTING it--"You pay us $10 per share for USE OF the stock for 3 years. We'd rather KNOW we have $1,000 NOW than MAYBE collect $4 per year. Or maybe they gave use of the stock in exchange for a loan.
If the stock goes DOWN in value during the loan period--it would have decreased anyway. If it INCREASES in value, the lender gets back shares worth more. The lender cares about the NUMBER of shares, not whether they are the SAME shares loaned. If you lend somebody $100 in $20's and they give you back $10's, you don't care.