Hello @Makkah Mohamed ,
I believe you are referring to Azure DNS delegation.
You can refer the below articles for more information regarding same:
https://learn.microsoft.com/en-us/azure/dns/dns-domain-delegation
https://learn.microsoft.com/en-us/azure/dns/dns-delegate-domain-azure-dns
As far as the downtime is concerned, you might need to wait 10 minutes or more after you complete the delegation, before you can successfully verify that it's working. And it can take a while for changes to propagate through the DNS system. So there will be some downtime that you need to take in account.
Kindly let us know if the above helps or you need further assistance on this issue.
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