A file that lives in
/var/www/discourse/plugins/plugin-name/public/folder/a.txt
will be in the URL at
https://discourse.example.com/plugins/plugin-name/folder/a.txt
A file that lives in
/var/www/discourse/plugins/plugin-name/public/folder/a.txt
will be in the URL at
https://discourse.example.com/plugins/plugin-name/folder/a.txt