Lower back during upper body bench exercises

When doing exercises on a bench (or ground for me), such as chest press and fly, what should the position of the lower back be?

As far as I know I think Daniel has said before to have it flat to the floor/bench like in core exercises, but I see a lot of people arching their back when doing upper body exercises.

I’d just like any advice on which is best? Or does it change depending on exercise?

Thank you!

Edited