I don't have much experience with curl. From what I understand, it's an old but constantly maintained command line tool. If you type curl https://www.google.com/ in your terminal of choice, you should get a text display in return of google's search page. That's if the curl command is installed on your system, which it most likely would be.

You won't be able to interact with it since it's in text but you can see how the page has been written in the HTML language before it gets rendered into the website you would normally see in a web browser.

When it comes to terminal commands, I find it helpful to do web searches using linux <command name>. For example linux curl and that will lead me to many sites that help explain the command and give multiple examples of how to use the command.

Once you get more experienced with using a terminal, using the command options --help or -h will give you information that could help you use the command. For example curl --help

There's also manual pages, or man pages that give a more technical look at commands within your terminal of choice. You can access them with man <command name>. Example: man curl.

In the case of federation, every platform that is using federation is using a communication protocol called ActivityPub. Simplified, it functions like email but instead of private emails, it's transferring public social media content. Microbloggers and threaded conversations can communicate with each other using ActivityPub but the information exchanged between the two platforms is slightly different. That's how we get quirks like this when two different ActivityPub platforms communicate with each other.

curl https://google.com/

If you run this, you will see the HTML for google.com printed to your screen.