I'm working on understanding top down design(well, at least bottom-up makes a lot more sense to me) and after watching a few tutorials I'm a bit confused about how to combine top-down with bottom-up.

I have several parts that I have created that act independently(connectors, knobs, etc...) and it seems these are true "parts".

In the tutorials I have seen they start with a layout and convert sketch entities into blocks which can then have a sort of assembly like process applied to them. The blocks are then turned into parts by what seems to be the exact same process to create "normal" parts.

Is there any way to use the parts I have created as the parts that the blocks are turned into instead of having to start from scratch again. If not then why not? What is the difference between the part design from a block in a layout from a normal part? (I do realize there is some connection between the layout and the assembly but not really sure what)

I can obviously use mates to mate a part to a block but this doesn't seem like the "correct way".

Thanks