Do you know what does "assembly" mean?
A: Component or end item comprising of a number of parts or subassemblies put together to perform a specific function, and capable of disassembly without destruction.
part = part file (creates new empty file in assembly workbench)
product = assembly file (creates new empty file in assembly workbench)
component = grouping (think this you have bolt, washer and nut. you can make a component this 3 item and you can use it every same hole connection.)
And component items do not appear in the bill of materials.