Buy & sell “XYZ” ---- buy & sell “ABC/UVW”

In present moment, I can keep my money in four types of currency: USD, BRL, BTC and UTH. The despite of the risk of each currency, I want to know to following, beyond theses currencies I can make trading with a lot of currencies through of a broker (EUR, GBP, JPY, etc, etc, etc).

So, my question is the following, what is the difference the a similarity between the following options:

  1. Buy USD
  2. Buy USD and buy USD/BRL by broker
  3. Buy only USD/BRL by broker
  4. Buy USD and buy EHT/USD by broker
  5. Buy ETH and buy EUR/USD

Do you understand? Today, exist a lot of different operations that can be executed singly or combined and the effects of each choice/combination are different!

So, someone already analyzed and studied case by case and got general theory about these exchanges? I’d like of more clarification!

Today, a part of my money is BRL, what’s the difference between buy USD and buy USD/BRL in a forex broker? This type of operation (buy and sell in fx brokers) is a CFD operation?

By the way, what’s exactly CFD?

Moreover, in a broker, I can sell USD/BRL. How is it possible? How can I sell something that I didn’t buy before?

And how can I replicate this type of operation without of a fx broker?

And why in a fx broker I need to give twos orders? “Input” and “output”? What happens exactly in the “input” order for be necessary an “output” order? When I buy USD out of a fx broker, I don’t need to give 2 orders, just one.