What are regiments in AOS?

For context, I play 40K and thinking of starting AOS with the new helsmiths army.

One thing I don’t understand is regiments in AOS. Is it just like 40K, where the hero unit is attached to the infantry unit. So the infantry acts as extra wounds for the hero?

But if that’s the case why can some hero units that have a a move of 5” join a unit that has a move of 12”(or the other way around).

So does that mean, that the regiment can be split so the higher movement ones can move ahead, or do they have to stick with the unit of the lower movement.