1. Bitcoin specifically is just a form of digital currency. Many people call it "digital gold". You don't really really need to join any groups to exchange for it. If you have an app that supports crypto wallets like Paypal, coinbase, cashapp etc you use the dollar to buy how much cryptocurrency you want. So if I wanted to buy a whole bitcoin i'd convert roughly $65k and own a whole Bitcoin. But you can also buy fractional amounts of it. So I can buy $5 worth of a bitcoin if I wanted to. The price of these currencies are always fluctuating based on volume, its usage etc.
2. This second question is harder to answer because there are a bunch of different cryptocurrency projects that do a lot of things. I own Bitcoin and Etherium so I can explain these. Bitcoin exists as a form of digital gold that's decentralized so it's not owned by any government or one person so people can sue it as a hedge against inflation. Etherium has it's own network that powers a lot of other crypto projects and has many use cases
edit: So the gas station you saw probably accepts Bitcoin as a form of payment. So someone could possibly use their bitcoin on their PayPal wallet or other crypto wallets to purchase items from the station.
