I get my domains from enom.com and they allow us to register and verify nameservers ourselves. Literally takes a couple of minutes.
Here is screenshot of the page where i do this.
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I would imagine that they probably call it "setting" nameservers, i would call it "registering nameservers". Basically you just need to be able to CREATE nameservers for magentobr.com.
No, You can leave it as it is. I think you have magentobr.com with dns1.stabletransit.com + dns2.stabletransit.com which is fine, no need to touch it for now.
Correct. On DNS ONLY server, no need to do anything (apart from make it secure but this is another story later on). On the main cpanel server, you need to configure the cluster.
Here is how to find the dns cluster configuration panel in cpanel.
View attachment 13072
On the cpanel server, you dont need to set anything in regards to dns. The dns for magentobr.com will be dns1.stabletransit.com + dns2.stabletransit.com.