Sat is short form of satoshi which the small unit of bitcoin. 1 BTC equals to 0.00000001 sat.
SATS is a BRC20 token that has nothing to do with bitcoin than using bitcoin blockchain for making transaction using bitcoin as fee. In short, SATS is an altcoins that make use of bitcoin network for making transaction.
I meant the altcoins SATS and 1000SATS
as far as i know it's just the way of some exchange simplifying meme coin with too much decimal because often time it confuses people and it also takes too much space to describe the price so they just bundle it and call it 1000sats or something, it also happened with other meme coin as well so it's not a new thing.
in a nutshell, it's the same coin but every 1000sats contain 1000 sats as the name implies.