Hi Karl!
Per the Microsoft "hacks" page: https://www.microsoft.com/en-us/microsoft-365-life-hacks/organization/how-to-manage-more-than-one-email-account - it states: "If each of your email accounts use the same email service provider, like Outlook, you’ll be able to create a unified inbox without downloading anything. In Outlook, tap the Account icon and select All Account to open a unified inbox with all of the accounts you’re logged into. If your email accounts use different email service providers, you can still try out a unified inbox by looking into third-party email apps that allow you to add all of your inboxes in one view.
Another option to streamline your email accounts is to set up automatic forwarding on your secondary accounts to redirect messages to your primary inbox. If your main reason for having secondary accounts is to protect your privacy, forwarding can be a good option. However, it doesn’t allow you to organize your emails as easily as when using a unified inbox."
SOOOOOOO... There absolutely IS a unified inbox, so long as all the accounts are "outlook" accounts ¯\_(ツ)_/¯