Okay cool so we're getting somewhere, there's just a few concepts you need to learn and this can be sorted out. So technically, your main nginx config file will be located at /etc/nginx/nginx.conf. Everything in etc/nginx/sites-available & /etc/nginx/sites-enabled will be vhost config files.
A vhost is short for "virtual host". In a land long long ago, before internet 2.0 & multi-core processors, you have a single host for every website. Someone came up with the technology to virtualize a host so a single server could work with multiples.
The nginx documentation is pretty huge, so instead of going directly to the source and getting lost for a bit, check out this article ( either read in full or at least scroll through and try to absorb the highlights) before continuing on:
https://www.digitalocean.com/community/tutorials/how-to-set-up-nginx-server-blocks-virtual-hosts-on-ubuntu-16-04
So you'll need a new vhost config file for every website you have. Only one vhost should contain the "default server" and each should have their root match the location of the website. Right now you have a single vhost file trying to serve multiple websites on /var/www/html. That won't work. If you have your site here, and flarum located another folder deep, I'd suggest cleaning that up and storing contents of your first website in /var/www/hml/{domain-name}. You'd need to modify the vhost root location directive to mirror this.
Something else to take note of is your server_name _; The underscore should be replaced with your domain name for the website. As is, the webserver interprets this as a catch-all for anything requested by a user that doesn't find a match in another vhost config file. This is likely the reason for the blank white page verses a webserver 500 error.
