I've made a large number of custom prints, and all of them were created using TinkerCad. It's an amazing toolkit, stupid easy to use but versatile. That is ... until something needs a tiny adjustment somewhere. That's when I feel it would've been neat to use parametric CAD instead.
I have spent many hours following Youtube tutorials for Onshape, Fusion, and FreeCAD. Tutorial shapes like a LEGO brick are fairly easy, although I admit that this kind of modeling is a sharp departure from the kid-friendly TinkerCad.
My problem is that I don't want to make simple coasters or keychains, but complex shapes like this one. It's a holder/mount for two different kinds of walkie-talkies that I use, and the blue part slides into a tray in my car's dash where it sits nice and snug.
Question: How the hell do I even get started modeling something like this?? There's not a single straight cuboid here. Everything is slightly wedge-shaped.
The way I do this in TinkerCad is that I build the hollow first: I made a 3d model of the walkie, a little oversized, set it be hollow, and drop it into the shape - that's the red or orange shells you see.
You use a real CAD program. The free tools can get you pretty far and are great for basic stuff. But as you're learning, once it's not basic, it's not a problem for a free shitty tool to solve.
It becomes a problem for a very expensive shitty tool to solve, like Solidworks.
Designing those things that slide into each other and everything on non perpendicular planes is child's play in SolidWorks. The slide in feature assuming they are mates is basically 3 button clicks and 10 seconds. Bam, done. Weird angles and planes, super easy.
You pay dearly for such ease. But that's how it's done. If you can't afford a trial or a student copy or a used copy, then there are ways. But a SolidWorks DVD from 15 years ago will do everything you want it to. CAD doesn't change much. And if you don't need super fancy 3D photo realistic renderings and the ability to import PCBs and thermodynamic simulations, than a 15 year old almost free copy of a powerful tool will beat any modern free tool.