Trucks carry 10 infantry or 1 artillery each. APCs carry 10 infantry each. Both the unit and the transport are involved in combat. When you raise the formation, it will build both the transport and the unit itself. For example, a 1000 strength motorized infantry would contain 10 infantry subunits (100 men each) and 10 truck subunits (10 trucks each). APC's fight at the front lines. Trucks hang out in the back, so they can't get attacked unless an enemy breaks through.
You can move a unit, then strat move it. But you can't strat move it and then move it.
I rarely use trucks due to the high manpower cost, but imo APC's are worth it because they WRECK soft targets. Putting artillery on trucks can be pretty useful though.
Trucks are sometimes better than strategic movement. One, the logistics cost of strategic movement can get pretty high. Two, if the units are on trucks, then they can move -> attack in the same turn.
That's probably too much information, but I hope some of it helps. Best thing to do is experiment yourself.
So when you say "Put artillery on trucks" this is when you group move 2 units together which one is trucks?
Afraid not, no. It's possible to create formations of motorized artillery that automatically include truck subunits alongside the artillery to tow them, but two different counters don't have any effect on each other for movement purposes. Group move has no effect on overall mobility, it's purely a convenience if you want every unit in a hex to move to the same place, and is no different from moving each counter individually to the same hex.
For what it's worth, you generally don't really need to know what the carry capacity of trucks are. The important thing is that when you raise a motorized/mechanized formation, the counters will have enough trucks/APCs to carry the rest of the unit and give them that much more mobility. Hypothetically if you got into a battle where you lost a lot of trucks/APCs without losing quite as many of the infantry/artillery you might get into a situation where you temporarily can't carry the entire unit until you get reinforcement trucks and the unit essentially becomes foot infantry, but that's rare.