You use it before the game begins. You’ll put
Ultra Magnus Armor onto Major Ultra Magnus, then shuffle the rest of your deck and draw 3 cards for your opening hand.
Let's say I have Major Ultra Magnus in bot mode and
Private Firedrive on the field, with Firedrive having 1 health remaining. If I attack with Ultra Magnus and trigger his bot mode ability, which causes Firedrive to die and I put the
Duo-Charge Electrostatic Photon Cannon onto Ultra Magnus, what happens? Does Ultra Magnus get the +3 attack for this battle? Am I able to scrap cards from my hand to increase said attack?
[Source: WotC]
Major Ultra Magnus will get the +3 attack, but since the long-name weapon was attached after he began his attack, the ability will not trigger and you will not be able to scrap any cards to add to your attack.
If I have Major Ultra Magnus with 16 Damage on him and my opponent has
Nemesis Prime with 15 Damage on him and I Attack
Optimus Prime - Battlefield Legend, causing my bot mode skill to trigger, does this cause Major Ultra Magnus to be KO’d and Nemesis Prime to be KO’d therefore no combat with Optimus?
[Source: WotC]
That is correct.
Before the game starts, find their related card, shuffle, then draw three cards for your opening hand.