It is common practice for nearly all those wealthy people relying on stocks. What I am interested in is the interest rate they get on those loans.

It's actually lower than retail bank interest rates. For example, if they get a huge mortgage, usually it is at least 100 basis points lower. This is because these people are considered extremely low risk.

I have heard it is pretty common for them to get 1 or 2% because the banks usually want to win their business.