Follow these steps exactly
Step 1: create a T2 or T3 unit
Step 2: click on the T2 or T3 unit
Step 3: a the bottom of the screen there is the build UI where you can queue units. You may think this is the factory UI, but it's not. It is the reinforcement UI. Click some units to order them as reinforcements.
That is literally all you have to do. Do it in a test game first and observe what happens after you follow the above steps. Lightbulb!
Factories will automatically build the units that you requested. The units that come out will automatically be part of the battle group led by the Zeus. As a result, they will get into formation around the Zeus. This means they will move from the factory to the ZeuS automatically. You don't have to do anything else. No rally points.
Obviously if you do not have a factory, air factory or advanced factory, those respective units cannot be built. I think the icons are currently not grey, which is a bug.
When more people start using this system, we'll be able to get more community feedback to help the devs make the reinforcement system better.