Explain how to make multiple parts into one part.
Procedures:
[1] Click [Boolean] on the [Feature] tab.
[2] Select [Union] from the [Properties].
[3] Select multiple parts to change one part with left mouse click.
The selected parts are added to the target Parts/Body.

[4] Click [OK].

[Boolean] work differently from drag & drop such as catalog block parts because they force the sum, difference, and accumulated shape of parts.
When you collect parts with multiple Intellishapes, it is a configuration where parts enter the part as follows.