How does one make units that come with other units

[Professor_Dyar]

Please forgive me if this has been asked before, but I looked and was unable to find it if it was.

Some units in the game are able to spawn additional units when they appear in combat. For example, the Moloch pretender comes with imps when he enters combat, even though said imps are not under his command as part of his squads.

How do you give this ability to a unit?

Also, more generally, is there any way to view the commands for each unit that tell you how that unit was made, so that if, in the future I find a unit with some ability that I like and would like to use, I can know how to apply it?

Thank you!