Nginx can be complex. My suggestion would be as follow:
Try the default config from your host. If it works, don't include the Flarum config files. If you customized paths, you'll need to block access to private files. This isn't something covered by the included nginx config so you will have to configure that manually anyway.
It seems your configuration is particularly complex because you use a subfolder and will also need to remove the public folder, which requires additional nginx rules to protect your private files as said above https://flarum.org/docs/install.html#customizing-paths
If the default config doesn't work and you're not familiar with nginx, I would suggest using another host. Apache is a lot easier for beginners and you'll find more people to help you with configuration issues.
Alternatively, if your host provides support, send them the links to https://flarum.org/docs/install.html#nginx and https://github.com/flarum/flarum/blob/master/.nginx.conf and see what they suggest you do.