In the Builder Base, you select which troops to train. But when you attack you can change the troops before you drop the first troop (even after you see the base you're attacking). I'm a bit confused as to how this works. Is there a penalty if you change troops, if not, what is even the point of training specific troops? How is the default army composition (i.e. the one you see when the enemy base first loads) chosen?
When attacking in versus battles, you generally already have an army trained. But when you click on the attack button and look at your opponents base, you can change your troops to any of the ones that you have unlocked, without any penalty.
This was done by Supercell, because there is no 'next' button like the one in regular multiplayer battles. So you have to change the troops depending on the layout of the opponents base then drop your troops.
The default loadout of troops is chosen based on your previous attack.
I did my previous attack with mass archers. So the barracks automatically trains up all archers in my army camps. But when i want to attack the next base I want to attack with archers and giants. So i change some of them to giants and then attack.
1) There is no penalty to changing the troops
2)The default army composition is trained based on your previous attack.
Hope this answers your question!
Training certain troops in your camps will always take the same time, regardless of what troop you select. Preferably select your "favourite" composition for the first time when you attack.
Changing your composition doesn't effect your loot bonus or how one person wins, after a game is over whatever army you used in that game is saved and set as the default one for the next game. To be more specific, the army that's displayed when you first choose a base is either what you set it to be in the camps or what it was last game (what you set it to).
You'll notice troops don't actually have a train time either, every single camp will take 60 seconds to fill, regardless of troop. (or number of that troop).