How do I unlock the iPhone that has a code and I don't remember?

You must have a computer, either Windows or Mac. If you don’t have one and can’t borrow one go to https://apple.com/retail and find a nearby Apple store, then make a Genius Bar appointment.

See this Apple support link→If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or watch this companion video ⬇️

You need a computer to restore it. Ask a family member or a friend who might have a computer that you can use.