FreeCAD 2020–
What?
Small, useful parts designed from scratch, mostly for 3D printing.
One example is a replacement gear for a car's air conditioner. I modeled it from zero and 3D printed it in PETG, and it works.
How I learned it
I learned FreeCAD in 2020, on my own in my free time while studying at ammattikoulu. At school we used Mastercam; FreeCAD was what I played with at home. I still use it now and then.