Hi @axiomatictor. It’s good that you already have a metamask wallet. Watch this link:
so that you can make the best of your metamask and UNISWAP interactions.
To answer your questions:
How are the altcoins stored?
All altcoins with ERC20 standard can be stored in your metamask wallet by just adding the token (altcoins) that you want. Watch this video to see how to add tokens in your metamask wallet.
Does it go to the metamask wallet no matter what altcoin it is?
To be honest, I’m not really sure if all altcoins (including those not ERC20 standard) can be stored in your metamask. But I’m sure that you can store NFTs in your metamask wallet. My advise is that, you should add the token that you want first on your metamask wallet before doing any swap. Check this video to have a clearer understanding of what I mean.
I hope this helps. Just ask if you still have questions and always do your own research before risking any money.