What is the difference between crypto, web3 wallet, and dex apps?

I convert crypto to a web3 wallet cause I thought it meant it can hold my money with new altcoins, nope, need to pay 100 dollars in gas fees just to convert back to ethereum. What is the point of this?

I want to know how knowledgeable the reddit community is about this and explain it like I'm a 5 year old please.

These things are important for me to know and maybe I will find a way to convert them because these funds feel stuck to my account and have no idea what the purpose for these are. Thank you.