How do you distribute the initial coins with PoS? Like who gets the first coin? The person who gets the first one will also get all the subsequent ones since he controls all the supply until he sells? How would that even work?

The tezos foundation for example made a crowdfunding some years ago. People who donated in the project received an amount written in the genesis block. The foundation started the mainnet in a beta phase with some initial validators. Holders started to participate in the PoS process until it was decentralized.