I have the latest release of CentOS7, and I did yum install firewalld earlier today. I've been entering some commands, but nothing advanced, and I would say that like 90-100 of it should be good.

I had some issues with sftp connection, but it was not related to firewalld. At this point it looks like firewalld is blocking httpd, as when it is turn on websites dont show, and when I turn it off they load almost right away.

I want to use this Linux installation for Apache web server, sftp connection (vsftpd), email (probably, I have not installed it yet, so this does not even count), and nothing else.

How would I make sure that firewalld settings are correct for this (unchanged, possibly only in the minimal way), and that everything works as it should, especially the httpd...

This is probably like two or three commands, and it is done. I just dont have that general understanding and perspective of how this works. I read quite a lot about it, but still I am not sure if I have it where it needs to be, settings-wise, at this point.

I mean, default zone - public, ssh enabled to there, in order for httpd to work - do this. What else would be good to do or check here?

Try firewall-cmd. To see what ports are open, use firewall-cmd --list-ports. To open port 80, firewall-cmd --add-port 80/tcp.

This is to the point, and it makes sense. I added the 80 and the websites work. I will make it --permanent, and so on, I read about that... I use sftp, ssh, and Apache web server, so I guess I need these two ports open, and nothing else.

Would I just leave it as is, and move on, or would there be any other settings / configurations that I could check... Like I said, I am particular about these things, as I want to figure out the best way to go with all the aspects of configuring these things, make notes, and based the further work on this. I want it to be 100% good / correct, so to speak, understand as much as I need to, before moving on to the next step.

Yes, I forgot about the --permanent option. Great that you found it.

Centos has a very tight firewall; by default, it only opens the ssh port and allows ICMP packets (i.e., ping). This is for a good reason, as you can imagine.

Only open the ports you must. If you run a web server, open the http and (if your web server also features secure http) https ports. If you run a mail server, the SMTP port. Set up your firewall for ftp if you have an ftp server. And so on. Don't open ports because you think you might need them some time; as you could see, it's almost trivial to open them at the moment you need them. Keep your system as locked up as possible.