The rovers there don’t gather rocks, but move around and look closely, including drilling into rocks to examine them. If there were fossils on the surface then they’d find them, but there’s other ways to detect existing and previous life, as well as an environment that would favor life, and that’s what they’ve been doing. I can’t give you a summary of all they’ve done, but NASA certainly has its archives and websites that will show the data as well as explain what some of it means. It’s an ongoing thing, lots of times discoveries come years after the actual photos or other data is collected.

The ice caps would be an interesting place to explore, but also a bit more risky since the ice (mainly water ice but also CO2) comes and goes with the seasons so adds more things for a rover to navigate. It also might make communication with Earth and collecting sunlight for energy a lot harder since it would be at a much higher/lower latitude on Mars.

I believe the closest we’ve got to fossils is some chemical signatures that indicate at least at one point there may have been some kind of very early microbial life. Maybe. Could have formed by other processes and the concentrations we see could be coincidence.

Fossils are hard to find and we’re really just at the starting stages of exploration.

Check out a documentary called “Good Night Oppy”.

It will tell you about the information the rovers gathered.