TinkerCAD is ok for simple shapes and basic functional parts. It works by adding or subtracting simple shapes together (cubes, cylinders etc) to make more complex shapes. It’s quick, easy and instinctive but anything slightly more complex than a dozen shapes grouped together and/or iterative designs quickly become a time consuming nightmare. It’s like trying to format a magazine in Word.

FreeCAD (or Fusion, OnShape, SolidWorks or any “serious” CAD software) use a parametric workflow. You start with a technical drawing by setting shapes, dimensions, angles and relationships (“constraints”), extrude or revolve this shape to create a solid, then continue by drawing another sketch on a face and adding more constraints, extruding this sketch, then… you get it. It has a much steeper learning curve, but once understood it’s much quicker and easier to build very complex shapes. Plus iterative designs are usually a breeze since everything is constrained together, so changing any dimension or angle in any sketch means the whole design will follow. It’s also trivial to add chamfers, filets, working with mirror and central symmetry etc. When designing functional parts, parametric design is the proper tool for the job.

They are vastly different, in tinkercad you mkstly operate with 3d primitives to make your model end result being an stl file. Freecad supports this workflow as well but their main loop is completely different, you draw your part in one dimension, then extrude it, then draw details in other dimensions and do operations on them, in the end you ideally get an model that can be easily modifable in every step. You can change for example hole diameter in your drawing and it will automatically propagate into model.