If you want yourdomain.com to point to your server, you need to update your DNS records with your registrar (unless you’ve delegated DNS to some other servers). This has nothing to do with Discourse.
When you mention people in Discourse, if you want them to know that you mentioned them, you type an @ and then their username, like @naupe.
If you want to install Discourse and Apache (and you have said nothing that implies that you do), you should follow these instructions. But, really, you should remove or disable Apache.
No. See the instructions I linked to or remove Apache. (Something like 1080:80 would be closer to what you are trying to do, but that’s not what you want to do.)
My advice:
sudo su -
apt purge apache
cd /var/discourse
rm containers/app.yml
./discourse-setup
Answer the questions. Don’t enable let’s encrypt. If http://nexus-matt.org is pointing to your Apache server, use that domain name for Discourse.
If your mail config doesn’t work, which seems likely to be your next problem, you can create an account like this:
sudo su -
cd /var/discourse
./launcher enter app
rake admin:create
BTW, the two major hosting services have free trials. For $0.36/day you can try it out on Digital Ocean. If you use a referral link (there’s one on my site), you’ll get a $10 credit on Digital Ocean, so you’ll get your first month free.