He's big, strong, and smart (brilliant strategist and tactician), and he's got presence. He's not quite Darkseid, but he would have been if Kirby hadn't left for DC.

He's pulled himself up to cosmic level by his own bootstraps more than once. He's got experience. Drop him naked on some backwater world and he'll probably walk into some local warlord's throne room, take over, and build on from there.

He knows where to find practically every single cosmic or near-cosmic artifact in the universe, or who to ask and how to pull their strings, and is on a first-name basis with most cosmic entities and elders of the universe, even if most of them don't like it.

How does he come to power? He decides what he wants, does his research, develops a plan (including fail-safes and fallbacks), and executes it, step by step, gathering whatever tools and disposable allies he might need along the way, mostly staying under the radar (when he's not cosmic the big players like the Shi'ar or Kree empires could spend a percentage of their GDP to curbstomp him if they wanted to, but even if they notice him he's generally not worth the trouble until it's too late).

And eventually he succeeds, an alliance of heroes (or maybe Squirrel Girl or Doom all by themselves) defeat him, he somehow comes back, rinse and repeat.

Probably nepotism as usual, his parents were famous eternals and his grandfather was Kronos, one of the most famous beings with the power cosmic and an early eternal.

You’ll get answers to these questions in Eternals 2 with Harry Styles some time this millennium.

IIRC he was known on Titan because of what his parents did, and also because he was purple.