How do you copy and paste code between WordPress sites?

If it is a plugin you just SFTP (see Filezilla) from the one site to another. Let’s say local host to Web Server. The plugins are in the dir
/wp-content/plugins/<my plugin>,

just FTP to the Destination, then enter to the Dashboardof Destination Plugin and Activate & Configure.
 Another way is with Git, you local installation is needed to update your Web installation , as two Git repositories. Transfer what is needed,

NOT TRASFER the wp-config.php  (includes passwords)