If you are facing 'git' is not recognized as an internal or external command with salesforce cli then what should I do?

GitHub Desktop is known as the distribution of Git that comes with a graphical user interface. It serves much of the same functionality as command-line Git, but if you desire to use this product to complete Trailhead modules, you'll be required to translate the commands provided for use in the CLI into actions in the GitHub Desktop UI.

GitHub provides instructions on how to clone a repository using GitHub Desktop.

If you want to use the CLI version of Git to solve git' is not recognized as an internal or external command, operable program or batch file, you'll need to make sure it is installed and configured appropriately for your OS. The main Git website is a good starting point that will provide download links for your OS. Note that while working with command-line Git, you'll have to take care of either authenticating with your username and password or setting up SSH, and you'll have to make sure (or your chosen installer will) that Git is in your PATH. GitHub Desktop abstracts those facets for you.