I have a collection NFT that I want to mint on OpenSea, and I’m confused about Ethereum vs Polygon

If I use the Polygon blockchain on OpenSea, does that mean the NFT doesn't live on the Ethereum blockchain, and instead lives on the Polygon blockchain? If someone buys it, can they not transfer it to the Ethereum blockchain? Will people who own ETH on the Ethereum blockchain still be able to buy it on the Polygon blockchain? Sorry for all of the question, I know next to nothing about Polygon. I was only looking at it as an option because I wouldn't have to worry about gas fees.