The key thing to know in FreeCAD is that you almost always want to use the "Part Design" workbench. Then the workflow for creating a part starts with creating a "Body". The "Body" represents a single solid. The major features of your part are created with 2-dimensional sketches (either on the base planes, or on flat surfaces of the part, or on datum planes that you can define). The sketches are then "Padded" up to form a solid bit or "Pocketed" down to form holes in the solid bits (although there are other operations as well - Pad and Pocket are the most important).
Knowing that much should be enough to help you mentally plan your part, and then looking at the tutorials should be enough to learn how to use the UI. If you are reading this and are having trouble with FreeCAD please feel free to email me (address in bio) and I will try to help you.
I love FreeCAD and I wish it was the "default" hobbyist CAD instead of Fusion360.
Blender is polygon-based 3D modelling with (pretty much) free-hand drawing and no constraints or dimensions.
FreeCAD and Blender can both export to many of the same formats (notably STL, for 3d printing), but their internal data representation, and supported workflow, is completely different.
FreeCAD is more appropriate when you want mechanical parts, and Blender is more appropriate when you want something artistic like a figurine.
Blender has ability to do simple constraint, but it's not its primary uses. FreeCAD can also be used the Blender way but the software will warn about being under-constraint. Both FreeCAD and Blender can export to STL file (used by CAD softwares), but Blender's STL usually need to be scaled to real world measurements. In case of 3D printing, this STL file can be imported into a slicer software such as PrusaSlicer, which outputs GCode to control the printer (e.g. X, Y, Z coordinates, extrusion temperature, etc.)
[1]: https://wiki.freecadweb.org/Tutorials
[2]: https://wiki.freecadweb.org/Toothbrush_Head_Stand