I find knoowingbthe basics helps.
I use blender. Its not perfect for 3d modeling. But I knew it a little to start. Also easy to set up on linux.
But honeatly learning its scale odds. (Defaults 1m to mm) boolean modifiers and 3d print tools. (Allow testing for real world models and extra faces.)
Really is all i need to modify stls and make my own sruff.