Totally confused on DNS workflow

I've been using cPanel (Apache) for ages and now moving to CyberPanel/OLS for speed & cost. After learning them and struggling a couple of days with my Joomla! .htaccess etc I could get it up and running on a VPS and started to move my websites. When I hit some walls I had to take a look to DNS records and got confused. I admit I never understood DNS stuff completely, but the behavior on CyberPanel is a lot different than cPanel. I need some clarifications on the workflow.

Say:
1) My server is myserver.mydomain.net on IP 123.123.123.123 , I have CyberPanel & OLS up and running.
2) Config Default Nameservers => ns1.mydomain.net & ns2.mydomain.net pointing to 123.123.123.123
3) On the VPS's ISP I defined rDNS to point to this server and defined ns1.mydomain.net & ns2.mydomain.net as nameservers for the main domain.

So far so good. Next I started to create one "reseller", created the "website" (say domain1.com) and started to move the domain by pointing to ns1.mydomain.net & ns2.mydomain.net.

Now, whenever I look at the DNS records I see the following:

A mail.mail.domain1.com 3600 123.123.123.123
CNAME www.mail.domain1.com 3600 mail.domain1.com 0
MX domain1.com 3600 mail.domain1.com 10
MX mail.domain1.com 3600 mail.mail.domain1.com 10

NS domain1.com 3600 ns1.domain1.com 0
NS domain1.com 3600 ns2.domain1.com 0

SOA domain1.com 3600 ns1.domain1.com hostmaster.domain1.com 1 10800 3600 604800 3600 0

So:

1) what is mail.mail ?

2) NS records point to non-existent name servers. I would expect ns1.mydomain.net & ns2.mydomain.net at their place.
Similarly SOA record also point to non-existent name server (and mail)

This is different from cPanel and I would expect the following:

Registrar/ISP - rDNS & DNS to point to my server
My server runs a DNS Server
Every domain on the server uses the same DNS server.

So, with the above defaults do I have to create ns1/ns2.domain.tld for each domain?

3) I tried to delete-change NS and SOA records and replace the defaults with ns1/ns2.mydomain.net versions, now I lost external access to the website.

I read lots of documentation and forum posts to no avail.
I also read that there are no DNS templates.

Before these changes (and without domain1.com nameservers) the website was working (I wonder how). But:
* I had problems with mail server SSL (may be not related)
* MXToolbox and other online tools were showing DNS related errors (as expected from above definitions)

Please advise how to correctly set it up...