Like USDT which is the most liquid and offshore, all the others are just being issued by different companies. There are more stablecoins with lower market cap and there is also the DAI for example which is decentralized and is minted (created) by locking collateral in MakerDAO platform.
Almost all of them are running as ERC20 tokens but some of them are running in multiple blockchains like OMNI layer (bitcoin), TRC20 (tron), Algorand, EOS, Binance Chain etc.
These coins are actually backing the whole crypto market and without them I'm not sure if we would be above 200 Billion USD total crypto market cap now.