If you want to make dimensionally accurate and functional things parametric 3d modelling is the way to go. If you are new to this one of the important aspects is community support and tutorials for whichever CAD you choose.
That kind of pushes you toward Fusion (formally Fusion 360) and Onshape. Both have free versions with limitations. Fusion doesn't run on Linux so I ruled that out and picked Onshape. It runs in a browser and is completely cloud based. The major limitation is that anything you create in the free version is public and visible to other users. Not a problem for most beginners.
I found Freecad didn't have much information out there relating to learning but might be good if you know what you're doing. I didn't look what the learning environment is like in OpenSCAD.
I started with a youtube tutorial that introduced the basics of sketches, constraints and extruding shapes.
I think it may have been the first three vids of this series by Invent Box Tutorials:
There are lots of tutorials out there so have a search.
Next I did the Onshape inbuilt beginner tutorial which was ok. It is a lot of sitting through short vids where they talk about features and as the practical exercises progress they start assuming you know how to do stuff they've not introduced. This chap has vids showing how to do the exercises when you get stuck. Saved me from frustration!
This site is designed to provide assistance and instruction for any student taking mathematics in High School. It is very much a work in progress on my part and I hope you find the videos useful and helpful. Mr. Jacobs Update; currently teaching CAD and Engineering...exciting times. Mr. Jacobs
www.youtube.com
After learning the basics the most useful thing I found was doing the practice models on
https://tootalltoby.com/practice/ Start at level 1, work out how to do the model yourself then watch the video of him doing the model. Finally go back and do the model yourself again incorporating what you've learnt. You can use any CAD you want but he uses Onshape for these tutorials.
