What is considered illegal gambling and why is some gambling legal and other gambling not? And what if it is legal in the USA but not in an another country and vice versa?

Sounds like you are asking for a thesis paper and not a one paragraph answer. Your question is extremely broad.

I have no experience with this act, but a quick search produced many references--this page, for example: https://www.onlinegamblingsites.com/law/uigea/

It says UIGEA does not define the term "illegal gambling." When that is the case, you are left with a general obligation to follow commercially reasonable practices. Guidance from banking regulators and trade associations would be very helpful, and you've probably looked into these resources already.

thank you for your responses