Using New Outlook on Windows for professional communication and productivity
Copy inbox of each account to favorites then delete other folders in favorites. All accounts should open together.
That's not a solution at all, like we said multiple times here favourites just show the incoming email, that's not a good way to combine email at all.
I suggest to just move to another email client like Mailspring or Thunderbird (I think Mailspring works much better but a lot of people prefer Thunderbird).
And even if on mobile outlook work fine I did not like the idea to be tied to microsoft anymore so I found an alternative called FairEmail.
Both Mailspring and FairEmail work better than outlook in any way possible, both are open source, both are free.
Most people that care about basic function like "All inboxes" should just switch, people that don't care about that function should still switch because outlook sucks anyway.