[General] How to make a unit ally AI control upon being trained?

[SoberToaster]

as well as. A way to trigger a wall that blocks 1 side of an arena from another. so units won't target each other until this wall is triggered to disappear?

Basically I'm just trying to make a really "simple" (which is proving much more complex than I thought it would be, I'm pretty lost frankly) map where there is an arena, you build structures that can be upgraded multiple times, they train 1 single unit and explode. theoretically upon being trained you lose control of said unit and it stands in place to allow the player to position the units how they want but to not have direct control over them. then the wall in the mid of the arena disappears and both teams will fight to the death. it's a two player map.

any ideas?
note, my skill level is about a 1/10 in the trigger department.
I'm having fun making a bunch of custom units and stuff but the triggers go over my head really quick.