The command format will be scp files_to_copy username@server_ip:path_on_server. The first argument will be the files you want to copy. In this case, you are copying all of the files in the build directory. The second argument is a combination of your credentials and the destination path. The destination path will be the same as the root in your Nginx config: /var/www/your_domain/html
command - scp -r ./build/* username@server_ip:/var/www/your_domain/html