Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below:

find . -type f -name "*.txt"

This will list all files with the extension .txt.

The . at the start denotes the current directory. find searches recursively in all the directories below the given path. If you want the search to start somewhere other than the current working directory, specify the path, for example:

find /etc -type f -name "*.conf"

This searches the /etc directory and all its subdirectories for regular files with the .conf extension.