They did explain but it was not explained in detail so it might have been difficult to catch. MC is not powerful he is just a regular guy from our world. He was using Xiao Bai's powers. When Xiao Bai was fighting Madman Chu he couldn't use all his powers since he has to maintain the Origin Realm. In the second lifetime he summoned MC to make him some kind of conduit to use Xiao Bai's powers. This was risky since the MC could choose not to give back those powers. He could choose to wipe out Xiao Bai and take over Origin Realm. But he did give them back. Although I'm not sure if that means he is back to being a mortal or he still has powers. At the end it's mentioned he still has godly cooking skills so it seems like he still has powers. There are still some parts that are not explained properly but it was an enjoying story overall.

He put a mark on some of MC's items? Would it still be there considering he blows up everything he owns? And for the stuff he can't blow up he refines them into artifacts so that he can blow them up too.

All he does with his artifacts is blow them up. 8th grade, 9th grade, Divine Artifact he just blows them all up. What's the point of making thousands of artifacts only to blow them up. Might as well make bombs.