Its a mess.
It really doesn't have to be. If you have, say, 5,000-ton formations as your basic type (companies or what have you), and you decide to have your basic infantry battalion consist of 3x Infantry companies, 1x Artillery company, and the HQ company with whatever else you throw in there, you can order the artillery company to support one of the infantry companies and it will work just fine.
If you on the other hand decided to, say, split the artillery company into three batteries of 1,600 tons each so that each one could support one infantry company, you could feel free to do this, but there is functionally no difference in terms of the effect on the battlefield in the actual ground combat. There is not even any significant effect from having formations "to protect the Artillery units" - Aurora simply does not simulate the tactical event of an artillery company being outflanked and overrun, if the artillery are shot at they simply take some damage in the same manner as every other formation, but by being in the support or rear echelons they are fired at much less frequently.
So if you just use the 5,000 ton artillery company and in-game assign it to support one infantry company, the mechanical effect will come out to be basically the same. Nothing stops you from role-playing this as an artillery company which is supporting all three infantry companies in a tactical sense, in fact with Aurora most players find that some aspect of the game mechanics or another does not fit their headcanon and choose to ignore it when role-playing. In general this is probably the more enjoyable approach to Aurora, to work with rather than against the game mechanics and simply imagine that everything works the way you would like it to - in a game where major planetary invasions usually require several
million tons of ground troops it is not too difficult to pretend that the artillery in a single company are coordinating with each infantry company on the tactical level even if mechanically they aren't quite doing this - this detail turns out to be very far removed from the actual ground combat effects that you see on the whole scale of the fighting, so there is nearly infinite room for imagination.
In general with Aurora worrying about how to get the exact tactical or operational behavior you want is only going to cause frustration. The magic of Aurora is that the game is played on the strategic layer, which gives you as a player endless freedom to imagine and roleplay how the tactical events took place.