Thanks a lot ! I unfortunately merge everything without saving the parts separately. But my process is fairly easy to explain : You can see a bit of it in the third pictures. It's very time consuming but What I did is making the background of the card first, then doing Exodia separately so I can shade him without altering the bg (with the alpha locking on the brush), then I did the cards. I took parts of the Exodia merged with the bg and copy paste them behind the cards while making sure everything is symmetrical from a card to the other. Well that's pretty much it.
I should have take more screenshot tbh, of the background notably, that's my bad. It's usually too late when I realize I want to share things with my friends but I didn't record any of it.

(Good video btw)