Unified Inbox in Outlook 2020?

So far, it seems to behave exactly as a unified inbox.  Sent and deleted mail all goes to the same folders under the "unified" email address.

You could set up a dummy email, but it would only be to help your thought processes, not to manage the account.  Because I was experimenting, I used a generic address that I'd never used for anything, but you can still set your default email independently (as the "from" when you click an email link, for example), and Outlook still replies from whichever address received the message.  As far as I can tell, you could pick any email you have, for any reason that makes sense to you.  I could make an argument for using your default email or maybe, like me, you've got an old email that you keep mainly to use to fill in for forms that might make you a spam target or one that just doesn't do much.

 They all just go straight to the chosen inbox.

Interesting.  Does Sent mail and drafts also get unified into this "account"?

I assume we should create a new dummy email address to be the unification one, not an existing one which might get confusing?  What is your default maibox?  Mine is Microsoft 365 account so that all my devices (mac, ipad, iphone, windows pcs etc) get the same synchronized calendar and address book (I use my exchange account as the glue between all these systems, but I cant use outlook on windows with it ironically due to the missing (for 15 years) unified inbox/sent mail/drafts etc.

And I thought I had a lot of email!  I don't think it is technically forwarding messages, in that new message are not actually delivered to their "home" address, if you will.  They all just go straight to the chosen inbox.

 At the bottom is a "Change Folder" button that corresponds with each email address as I click on it.  From what I can tell, I can use this button to send the mail from all of my email addresses to a single inbox.  So, I pick my default Outlook address, which doesn't receive any mail, and all of the emails from all my addresses go to that inbox.

I dont know what a "folder" is in this case, but it sounds like you are forwarding a copy of all your mail to a single inbox?  I have may be 200 thousand emails in my 20 or so inboxes (I get more than 100 a day) , I would worry that this cause outlook to break (it used to fall over when I hit the 10k barrier).  I have ordered a Mac to replace windows (just so I can get the unified inbox), but with covid it has been delayed by some months.

I used emClient for a while some years ago (the first time I tried to break from Thunderbird).  I don't remember why, but I found it just wasn't up to what I needed.

I'm not 100% sure I understand your question, but I think I can answer it with "I don't think so."  The unified inbox and sync with mobile as described above all happens within Outlook. Other mail clients will still go straight to the server.  So, if you set Outlook to never delete messages from the server and instead treat some other email client as your home-base, so to speak, where you give emails their final deletion, the messages will be there for those other clients to grab no matter what you do in Outlook, mobile or desktop.

At the moment, I'm still running Thunderbird alongside Outlook.  I've got both set to delete messages from the server only when they're deleted from the client, so the only way they interfere with each other is if I delete a message from one before the other has downloaded it.