So without escorts, they would fight exactly like any other convoy right? Hit by a torpedo and some fraction of your troops are going to the bottom, like any other cargo.
As for the escorting question, as far as I can tell, you put fleets on convoy escort duty in the naval regions that you will be moving troops. According to the naval DD, they will "be very likely to be protecting convoys in the region when attacked." There doesn't seem to be a way to say 'protect this particular convoy' and not the others. So just make sure you have way more escorts than the enemy has raiders in that zone, or be prepared to roll the dice. Other ships will try to help if the convoy is attacked but may not get there in time.
Edit: From the sub DD: "Troops being transported or during a planned invasion may be attacked leading to them being wiped out or severely weakened. You always want to be very careful and guard your transport routes before sending troops somewhere in war time." Note that you guard transport routes (ie seazones) not convoys.