So, FreeCAD. It's a beautiful hot mess. There's a 1.0 in beta right now that's bringing some much needed changes.

FreeCAD has a lot of parallel capabilities; it has an architectural workbench for drawing buildings, a Drafting workbench for more traditional 2D drawing, the Part workbench for a weird kind of boolean approach, and the Part Design workbench for a more typical sketch-and-extrude parametric modeling workflow like Fusion360, Inventor or OnShape.

The workflow is you create a sketch and draw a 2D shape, and then extrude (FreeCAD uses the word Pad) it into 3D space, then you can draw further features on that to design the shape you want.

The basis of how it works is somewhat unintuitive at first. "Parametric" means you draw using rules. There's a piece of software out there called OpenSCAD that is a very pure implementation of this because you "draw" by typing code in a kind of programming language. FreeCAD lets you represent rules by drawing things with the mouse. Rules like "this is a straight line. It is parallel to the X axis. It is 5cm long. The leftmost endpoint is 3cm from the X axis and 4cm from the Y axis." There's only one way to draw that line. Those rules may be called Constraints or Dimensions. The powerful part is you can later change one of the rules, like "Did I say 3cm from the X axis? I meant 4cm" and it'll redraw the whole part for you. Get your head around that concept and CAD software will unlock.

The UIs are different, but the general concepts are similar for FreeCAD, OnShape and Fusion360, sometimes tutorials for one will be useful for learning the others.