Forex is a decentralized market, so exchanges like in crypto don't exist. There are large pools of liquidity (ECNs) that you could say are similar to exchanges though.

In terms of what markets to trade, I would suggest to go with liquid markets first. Obvious choices would be Gold, Oil, Wheat, Corn, Silver etc...First to try are the big two, Gold and Oil. Once your start to go down the rabbit hole are all sorts of things you can trade on futures markets like Azuki beans for examples.

There is no fractional reserve afaik, the money in your broker account is supposed to be segregated. In practice we know that's not always the case :)