How does borrowing against BTC work?

I’m trying to understand how borrowing against Bitcoin works in practice.

How much can you usually borrow relative to your BTC value? Is it a fixed percentage across platforms, or does it vary a lot by lender?

What happens with private keys during the process? Do you keep custody, or is the BTC moved to a different wallet as collateral?

I’m also curious whether this is generally considered a smart move, or if selling BTC outright is usually the safer option.

Lastly how long does this whole process typically take, hours, days, or longer?

Appreciate any insight from ya'll.