The basic function of crypto is that you can buy cryptocurrencies with dollars and sell cryptocurrencies for dollars. I suppose there are other functions? But the main thing is that if you think Bitcoin will go up, you spend some dollars to buy some Bitcoin, and then if it goes up (or doesn’t), you sell it for dollars.
If you like crypto a lot — or if you are an institutional-ish crypto trader — you will do this a lot, and you may find yourself frustrated with the dollar side of things. Crypto trades globally, 24 hours a day, seven days a week; you can use smart contracts to send crypto automatically, and sending crypto is generally a permissionless lightly-regulated activity. But if you want to buy crypto with dollars, you need to use the US dollar financial system, which can feel clunky to you, a crypto native. You will probably have to send a bank transfer, but the banks are not open 24/7, and some of them might raise annoying questions if you try to transfer money from your bank account to buy crypto.
There are solutions. One solution is that you take your dollars, you deposit them at a big trustworthy crypto exchange, and then you use the dollars in your exchange account to buy and sell crypto. The exchange holds a bunch of dollars and a bunch of crypto for its customers, and when you buy crypto the exchange deducts some dollars from your account and adds some crypto to your account, and vice versa when you sell. There are problems with this solution. The biggest is of course that sometimes the big trustworthy crypto exchanges aren’t, and they lose or steal your dollars. But another issue is that, when you deposit your dollars at the exchange, it needs to deposit them somewhere. It needs to keep customer dollars at some crypto-friendly bank that will give them back on demand.
Another solution is stablecoins: Instead of keeping your dollars at a bank, you turn them into crypto dollars by buying stablecoins that are meant to always be worth a dollar, and then instead of buying and selling crypto with dollars you buy and sell crypto with dollar-denominated stablecoins. But here too you have to trust the stablecoin issuer, which is not always a great idea. (Other stablecoins are algorithmic and that’s also risky.) And the stablecoin issuer needs to put the money somewhere, so again there is a need for a crypto-friendly bank.
Another solution is bank fraud, but don’t do that.