How is it that the pros connect the ahu?

Hello I have yet another question about how the pros hook up an air handler to the plenum when the ahu is located in a small closet for example. In 2020, I watched them briefly install my girlfriend's parents new system, but didn't get to see how the guy sealed the back side of the ahu to the plenum. On the front and sides, he used foil tape and then put mastic over it. Is this correct? On the back, I didn't see what he did after he cut out strips of duct board (in guessing the old ahu was taller) . How can I be comfortable air won't leak out the back? How do you guys normally do it when there's a wall right behind the ahu? I've looked for pictures/similar info online, but found nothing. Thanks

source https://hvac-talk.com/vbb/threads/2240388-How-is-it-that-the-pros-connect-the-ahu?goto=newpost