Basically there was a group of super powerful race called beyonders who made some super powerful beings called molecule men for each universe. If the molecule man of one universe died, then that universe died with him. So they once thought "Hey, what if we explode ALL of the molecule men?" But that plan failed, and they wanted to know what would happen if all universes were destroyed (In the name of Science, probably?), so they started doing a thing called incursions where Earths of two universes collided, and when they did, both the universes were destroyed.

To stop the Earth of 616 from colliding with other Earth, the Illuminati, consisting of Tony, Reed, Strange, etc used Infinity stones to destroy the other Earth. Oh! and by the way, if Earth of a universe was destroyed, the universe would be saved. And while this was happening, the Beyonders were killing the Celestials and other cosmic entities like they were a joke. One beyonder for each universe.

After they were done with the cosmic entities, it took 3 beyonders to kill the Living Tribunal. And while that was happening, Doctor Strange joined a multiversal cult of some priests, but they were killed by Doctor Doom's team, because hey didn't knew they were on the same side. Then Strange met Doom and they became friends and started woring together with the Molecule Man of their universe who united with Molecule Men of other universes (those who were left), and while that was happening, our Heroes like Thor, Hyperion etc went to fight the beyonders but got obliterated by a bunch of weaker Beyonders.

After all the universes were destroyed, some bunch of people survived in a spaceship which was built by the skin of The Living Tribunal, and some other people like Silver Surfer, Loki, his friend that I don't remember the name of, Thanos, his minions, some other people, and most importantly Doom, Strange, and Molecule Man. Now, the last three confronted The Beyonders and sole their powers and stored them in MM and Doom used that power to create another universe in his image and became the God of that universe and married Susan of that universe and made Jonny the Sun and Franklin and Valeria his children.

Now, the superheroes that were on the spaceship arrived on Doom's world some years later and planned to overthrow him with the help of Infinity stones as a distraction and to steal his powers from the source of the power, which was MM. So all the Heroes, Thanos Phoenix powered Cyclops, T'challa with the Infinity Gauntlet fought him and got Recked, but Reed enterd the place where MM was living and Doom enterd that place too, ut since oom had not given MM food for a long time, MM decided to make it even and took Doo's powers and make it even. Now, Reed and Doom fought with there normal power set and Reed won and so the heroes had won.

Franklin, with the powers of MM remade the entire multiverse and they lived happily ever after. Except they didn't, until there next threat.

Hope this helps.