Wow.... I'm also trying to find an easy way to do this.
The Outlook app on my iPhone and iPad has All Accounts, which shows all emails from all 7 of my accounts (2 of them being Gmail addresses). It's perfect in every way. Simple to use, and everything is easy to find.
I have Outlook 2021 (as part of Office 2021), and there seems to be no easy way to do it. I've Googled the hell out of how to have a Unified Inbox. It seems I have to merge accounts, or create rules, or run a VB script to make it possible. That's a lil difficult for the average user.
The "new" Outlook app for Windows which apparently will replace the Windows Mail app in 2024 doesn't seem to have the functionality either.
The current Windows Mail app doesn't have the a Global Unified Inbox, but at least it's simplified, in that it shows each of the Accounts up top in the left pane, and Folders below in the same left pane. That's still usable for me.
Whereas, the "new" Outlook app is much like the Outlook you get with Office 2021/2023/365, it just shows all the accounts, and you have to click the > to expand the folders within that account, and it becomes a massive pain in the rear if you've expanded all of your email accounts, you then have a lot of scrolling to do with your mouse or cursor keys to find/move items.
Not only that, but the "new" Outlook Windows app includes advertisement messages, the background image is gone, a lot of the functionality is gone, and rather than clicking on an email account and seeing all folders, it's a collapsible set of folders. It looks ugly, feels clunky, and quite frankly, is a pain in the rear to use.
Why can't they make something simple to use with one Global Unified Inbox as an option for the Outlook app and Outlook 2021/2023/365 like it is on the iOS version of Outlook???