as you can see, the conf files already hold an include directive that allows for custom configs. you just need to put it in /home/user/conf/web/sub.domain.com/ and name it nginx.ssl.conf_something so it matches the include.
after that run nginx -t or try to restart and check journalctl output if it fails.
depending from whereever you copied whatever is in that conf file you most likely need to adjust it to your installation.
use your administrations skills and linux powers for that! good luck.