Hi @KeN L3W
Thanks for reaching out to Microsoft Q&A
The way that mail related attributes are populated depends on several things like licenses assigned, source attributes set, etc. This document explains how it's synced and all of its dependencies - https://learn.microsoft.com/en-us/entra/identity/saas-apps/servicenow-provisioning-tutorial#step-5-configure-automatic-user-provisioning-to-servicenow
The users you are seeing as OnPremises Synced "Yes" are the ones that came from your AD. The ones set as "No" are either users created directly on Entra ID or Guest users
In regards to editing the user's properties, you can only do that on your onprem AD for the synced users, you can't edit them on Entra ID.
Let me know if you have further questions.
Thanks,
Fabio