Uploading a file to the root directory of a WordPress site is easy. Simply upload the file to the root of your WordPress site, and then use the wp_upload_file() function to specify the name of the file you want to upload.
WP_upload_file() accepts a number of arguments that determine how the file will be uploaded. You can specify the file type, the file name, the upload location, and the file size.
You can also specify the permissions the file will have.
WP_upload_file() will return true if the file was successfully uploaded, and false if there was an error. You can use the wp_get_file() function to get information about the uploaded file, including the file name, size, and file type.