Some questions

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Hello, first of all I clarify that I am using the google translator. I apologize for that.

After some time of knowing this great forum system, I decided to explore in these totally new places for me. After spending many hours and days trying to understand its operation to install and then be able to use a domain with ssl certificates, I have many other questions that I will try to tell next:

A - From both trying things I do not know if I did other things and now have problems with the security of the site and the server. Some of the things I remember:
     1 - I added a non-root user.
     2 - Change the authentication of public keys (although I do not know how it works)
     3 - I activated (I think) the firewall
      What do you recommend me to do?
B - After so much work with nginx and trying to understand the configuration steps, now entering through my IP address is impossible. Is this really like this?
C - In this guide: https://docs.nodebb.org/configuring/proxies/nginx/
What are the files that must be created and / or edited?

I hope you can understand me and help me.

Thanks so much for reading!