What 3D modeling software do you recommend? Any open source 3D modeling apps worth looking at?
Open source? FreeCAD, OpenSCAD, Blender.
Personally I use OpenSCAD for things where being parametric is a primary goal.
Tinkercad isn't open source, but it is free and CSG modeling is good enough for a lot of practical things.
Fusion360 isn't free or ope source, but it is still "free" for hobbyist use (just have to make sure that's the download you pick) and my go-to for anything more complex than I'd want to tackle with Tinkercad.