"My pleasure" = You're welcome, but even more polite in my opinion "It's been a pleasure" sounds like they are about to say goodbye

Informally, they both invoke a similar feeling of gratitude and happiness. For example: Professor: Thank you for volunteering at the department's charity event Student: You are welcome! It has been (a pleasure/my pleasure)

It depends on the situation but for casual everyday talk or even for most business situations its barley ever used. When it comes to "its been a pleasure" The contexts that come to mind are usually hospitality industry usually high end- think expensive dinner. That is something a fine dining waiter would say or chef showing gratitude. Or possibility wrapping a business meeting with a new client of some sort. "My pleasure" basically the same. I also experience this in dating when some one is new and trying to impress or simply someone that has values/dignity.

“It’s my pleasure” is a response. “It’s been a pleasure” is a statement that can just be said to someone, as already mentioned it is usually used when leaving.