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.