If you create an erc20 token with 20 million supply, how do you distribute it? I understand doing an airdrop, but that would maybe only give away 1 million. So I don't get how these new tokens are trading on exchanges...how did they distribute their token so quickly?
You need to understand that it is very easy to create an erc20 token, as long as you are able to handle the etherum blockchain smart contract.
Seeing that you are a newbie, you need to first learn how etherum works, how to create a smart contract, and also how to conduct an airdrop.
After understanding this, you can further figure out how to send tokens to multiple addresses using multi send.
Understand the blockchain, and do your own research