You can create a new part in the context of an assembly. That way you can use the geometry of other assembly components while designing the part.
You can also create a new subassembly in the context of another assembly. See
Inserting a New Subassembly.
Before creating new components in the context of an assembly, you can specify the
default behavior for saving the new components either as separate external part
files or as virtual components within the assembly file. See Saving New In-Context Components.
To create a part within an assembly: