Some thoughts from my side:
- Don't worry about which workbench to use, start with "Part Design"
- A "body" is a single solid object - think something you can 3d print for example, or a single piece of metal etc.
- You normally create a body by performing an operation on a "sketch". For example, you might sketch a circle, then perform a "pad" operation of 10 cm to create a 10cm high cylinder.
- A sketch is a 2 dimensional set of lines and arcs that you set "constraints" on. For example: the centre of this circle is at the end of that line, this circle has a radius of 20 mm, this line is horizontal, that line is 100mm long etc.
- Once you need to put bodies together, you can put them inside "parts" - a sort of group basically, that can be moved and rotated, and can contain both bodies and other parts.
So, for the box and lid example: you might create a sketch in a body. containing a square. Let's say you make it 100mm x 100mm. Then you pad it 50mm. This gives you a body 50mm high.
Then you create a sketch on the top surface of a smaller square, and "pocket" by 40mm to cut the interior of the box (attaching sketches to faces is not the best approach for more complex designs, but worry about that later!).
Then you create a second body, padding a square by 10mm to create the lid.
You can use "edit" / "placement" to move the bodies around.