Downloading a file is a two step process.

Get the File Summary object

Make a request to the Files endpoint using the HTTP GET Method and including the id of the file. This will return a file summary object containing information about the file.

Download the File

Once you have obtained the file summary object, you can now use the value of the link property to download the file. Note: that this is a temporary link, the LinkExpiration property indicates when the link will no longer be available.