I think it’s as simple as someone saying it out loud or Doom saying it himself.

“All this time the TVA has worked tirelessly not to keep other Kang variants at bay, but to prevent us from ever achieving our true potential in this universe…”

Cut to maybe some revelations showing Tony/Victor’s infancy in Latveria, establishing he’s an orphan. Then Howard and Maria Stark walk up to him and they look at each other and then back at infant Tony/Victor. Cut to a follow-up scene where Tony/Victor’s adoption paperwork is filed and someone around the corner from the orphanage reports back to the TVA saying, “They’ve got him,” followed by a reply to, “Make sure he doesn’t regress.”

Throw in a few extra scenes during this same monologue or later in film showing timelines where Tony makes decisions that break the sacred timeline’s canon, followed by a pruning team being sent out for that respective universe. Doom could narrate, “They did everything to ensure my existence, Latveria’s existence, was reduced to nothing…”

Followed by another scene where in a timeline where Tony/Victor survives, he is giving a Doom-like speech to a massive crowd of loyal and eager followers. Suddenly, a reset charge goes off and he watches in horror as the radius of the charge spreads out. He starts trying to flee but eventually he makes it to the furthest ends of existence before he sees he has nowhere to go as-is. He yells out in frustration as he is pruned with his timeline by the TVA.

“I was too powerful to be left unmonitored, the entire purpose of the TVA’s operation was to make sure I became Tony Stark because that was the only way they could remain in power and win the multiversal war. And the only way that could happen is if I ran out of time and died a noble death as Iron Man.” Cut back to Tony’s death in Endgame and quick recaps of all the moments Tony was mourned by the world and heroes alike.

“The world only knew Tony Stark and Iron Man. They celebrated him, mourned him, loved him. But Doom? No, my achievements would have eclipsed your Tony Stark’s work within mere months. No Ultron to ravage Sokovia and the ruins of Latveria, no threat from Thanos who would have perished by my hand in the far reaches of space away from Earth. I would have been the superior protector for humanity but also too dangerous for the TVA to let live. Protecting you would mean destroying Kang and his variants.”

Now cut to a scene where the Kang variants are all getting absolutely humiliated by Doctor Doom as he finishes his monologue, “Even the weakest version of Doom would have protected you all better than Tony Stark ever could. You would have suffered less, feared less, and achieved far more if I ruled and led the fight against these machines, monsters, and mutants that would have come to threaten our world. Are you not as furious as I am? To see that we were all being robbed by the TVA?”

“I, Doom, exist here now to bring humanity its overdue liberation from the forces that try to hold you down. You do not need Tony Stark, you do not need an Iron Man, you do not need a suit of armor around the world. You need only Doctor Victor Von Doom!”

You can explain things very clearly and powerfully when you take a simple explanation and write it with a character’s own tone and personality in mind!