A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
Two different options. You can populate them as contacts in the GAL or even have a separate Address List that can be selected from the Address Book dialog. More info about this scenario: https://learn.microsoft.com/en-us/exchange/address-books/address-lists/address-lists
Alternatively, you can populate them as items within a Contacts folder, which you can then share with whoever needs access. Or even populate them directly in user's mailboxes. For that, you will need a programmatic solution, based on the EWS or Graph APIs. Here's a sample one: https://github.com/gscales/Powershell-Scripts/tree/master/EWSContacts