Here is a great tenant name checker
https://o365.rocksThe tenant name must be unique and it is the basis of several things including your SharePoint url. Basically, the rule of thumb is to get it as close to your org name as possible (I.e myorg.onmicrosoft.com) or something approximate to the use of 365 I.e myorg365.onmicrosoft.com
The biggest thing to understand is that this is permanent. You can’t change it after and if you want to you have to move to another tenant.
There are several good articles out there on this including
https://techcommunity.microsoft.com/t5/Office-365/Implications-choosing-Tenant-name-for-Office-365/t...Hope that answers your question!
Best, Chris