You can use OnShape for free as long as you're OK with the models being publicly visible. I find that fine for learning and personal projects.

I've dabbled with OnShape, FreeCAD, and SolveSpace, and of them SolveSpace is the one I've ended up using the most. OnShape was nice, the GUI was pretty intuitive, I liked the way it worked, but I just feel weird trusting anything to a free plan on a cloud service. I don't really mind the public part, but it always felt tenuous that the plan would remain free so I didn't really feel like I could trust it long term.

FreeCAD was complicated and opaque, I never really put in the time to learn it, it just felt a bit clunky, but I keep meaning to come back to it.

SolveSpace seemed a bit mysterious at first, but just a bit of learning and I found myself pretty comfortable with it. It's not nearly as fully featured as some of the others, but it clicked well for me.

SolveSpace and FreeCAD are both FLOSS software.