Lets talk about Fu in the alt-Universe.
In Shadybug and Claw Noir's universe, the Supreme holds nearly all the miraculous. But if the characters from Ladybug and Cat Noir's universe are still in that alternate universe, then that must mean there was also an order of the guardians that protected the jewels, so the Supreme was not the original holder of the miraculous, it was still the guardians.
However, something changed the course of events for the guardians and the miraculous in that alt-universe, and it must have been alternate universe Fu. Fu was at the center of the temples disappearance nearly 200 years ago, leaving him the last remaining guardian on earth to protect the miraculous left in the miracle box, he spent his whole life protecting the miracle box so the jewels would never fall into the wrong hands, but in the alt-Universe, things may have gone differently, not just for the order, but for alt-Fu. In the alt-universe, the characters are still the same people, the only difference is that everyone made different choices, some of which led to drastically different outcomes in their lives. This is important because something happened on alt-Fu's side that also led to the Supreme obtaining all the miraculous and taking over the world.
The one major event in Fu's life that could have changed the course of events and resulted in losing the miraculous to the Supreme is obvious, it was the day the order of the guardians disappeared. Fu became the last guardian after his mistake led to the entire order being eaten alive by a senti he created. The senti, Feast, was born from the peacock miraculous Fu used in the middle of a trial where he had to guard the miracle box for 24 hours, without food or water.
Unfortunately, Fu's hunger mixed with the resentment he had against the order and the miraculous for taking him away from his family at such a young age, forcing him away from the life he wanted to choose for himself, resulting in the creation of a senti who wanted to eat the miraculous along with everything else around them. That anger Fu felt as a kid towards the order of the guardians is what convinced him to break the rules and take the peacock miraculous from the box. Still, what ultimately decided Fu's path in life was how he felt about the consequence of his mistake.
Fu was filled with immense guilt for being responsible for the deaths of all the people in the temple and the overall destruction of the order of the guardians. Acknowledging that guilt pushed him to try and take responsibility for his actions in the only way he felt he could, by fulfilling his caretakers final wish to protect the last miracle box and the spell book.
In the alternate universe, alt-Fu might have chosen a different path for himself. If alt-Marinette and alt-Adrien chose to hold onto their anger and resentment towards others, unwilling to move on from the painful events of their lives in order to find a better path for themselves, then alt-Fu likely did the same. Rather than feel guilty for causing the end of the order of the guardians, alt-Fu might have held onto his anger towards them and the miraculous. So unlike Fu, Alt-Fu didn't take on the responsibility of a guardian and never devoted himself to protecting the miraculous, instead, he might have chosen to take advantage of the immense power he now had.
The show hasn't revealed who the Supreme is yet, but its rather obvious at this point that the signs point to the Supreme's identity being alt-Fu. Its no coincidence alt-Marinette and alt-Adrien were still chosen to be the holders of the ladybug and black cat miraculous, they were chosen because alt-Fu chose them, just like how Fu chose Marinette and Adrien in their universe. Except alt-Fu had a different reason for choosing his holder, but more on that later. It was also no coincidence either that the Supreme's symbol looked near identical to the guardians, their similar because the Supreme was someone who trained under the order of the guardians, holding enough resentment towards them to take that symbol they were known for and re-create it into a version that looked like someone destroyed it.
Just like Fu, alt-Fu would have been in the rare position where he had an enormous amount of power in his hands, enough needed to even do something as extreme as taking over the world, he had all the miraculous, the grimoire and the knowledge needed to read the spell book, knowing what needed to be known about the miraculous, including that spell that prevented others from using Plagg and Tikki's wish granting abilities. Of course, someone in that kind of position wouldn't be able to rule the world on their own.
Betterfly said the Supreme didn't work alone, he had loyal followers comprised of the rich and powerful, people who would be selected by him to even wield a miraculous from time to time. Those rich and powerful followers were no doubt the members of the secret council Gabriel and Nathalie were part of, a council comprised of the wealthy and corrupt who were determined to find the miraculous so they could use the ladybug and cat miraculous wish granting abilities.
But while the secret council in ladybug and cat noirs universe is led by someone else, it wouldn't be all to surprising if alt-Fu came into contact with the secret council in his universe and completely took over, using their resources and expertise to help maintain his rule over the world. But lets wait and see what the show has to say.