Can you move 2 (with mech or tanks) onto a transport?
Can you move 2 (with mech or tanks) off of a transport?
I’m not really sure what your question is.
A transport can hold 1 of any land unit, plus one (non-mechanized) infantry. So it can have a tank/mech. infantry/artillery/AA gun/infantry and an infantry. It can’t hold both a tank and a mechanized infantry. If they’re being loaded in the same turn, it has to be in the same phase (both during the combat phase or both during the non-combat phase) and they have to be offloaded in the same phase as well. If they’re being loaded and offloaded in the same turn it has to be within the same phase (you can’t load in the combat phase and offload in that turn’s non-combat phase).
Edit: It seems that your post may be better answered below, depending on what exactly the question was.
Does this mean that a transport can hold 2 artillery? or is it 1 infantry and 1 tank/artillery/mech./AA gun/infantry?
No. It can hold one unit of any type plus one infantry. So it could have 2 infantry, an infantry or an artillery, an infantry and a mech. infantry, an infantry and a tank, or an infantry and an AA gun.