The API doesn't fetch anything. It's an interface, which enables you to fetch the data. You're interested in this endpoint: https://api.chess.com/pub/player/terabytecarter/stats. I've used your name, the name would be different for every player. It contains data in the JSON format (I suggest you open the link in Firefox, which displays JSON nicely).
If you want to automate the process you've described, you need to write a little bit of code: for each player, make a GET request for the URL (based on their name) and extract the rapid rating from the fetched JSON data. Then write the ratings of the players into a file. It'll likely be quickest if you copy & paste the data to the G sheet by hand then.
Edit: Do you have experience with any programming/scripting language?
I created a chess club at my high school. To ensure fairness in matches, I created a spreadsheet (Google Sheets) with all members' names and a column for Rapid ratings. However, it is obvious that manually editing this would be tedious. I know that the API can fetch Stats data, but I am new to working with this. If anyone is able to help, it would be much appreciated. Thank you.