Generally speaking, dating describes a less serious level of commitment before either person is ready to describe the other as a girlfriend or boyfriend. This is not a firm rule because the terms have no strict definition.

What is the difference between seeing someone dating someone and having a girlfriend boyfriend?

“Seeing someone” is exactly the same as “dating someone”. It is what you say in English when you have regular interactions (dates) with one person, but you are not their girlfriend/boyfriend. You can see or date lots of people at the same time or exclusively see/date someone.