@AngelBK

Here's what you asked, regarding QB Desktop:

You use Word. That is a program you and I both have on our computers. I create a Letter = the data file. I send you my letter, which you copy to your computer and use Word to work on it. While you are updating the letter, I cannot also be working in my copy of that letter.

Finally you return that letter to me, and I replace my letter with yours, since yours is the updated letter. Now I have the Updated letter, too. You cannot work in your copy of the letter, if I need to work in that letter. I would send you a New Updated copy of the letter, and I stop working in my copy of the letter while you are working on the updated copy I sent you.

Now, change the word "letter" which is the Data file; instead, that is QB Company Data file, the .qbw file. They Make a backup and send you a backup, which you Restore and use. While you do that, they cannot Also keep working, since you are going to make a new backup and send them the file you Updated.

You don't get to their Desktop. You get Their File and you swap updated files back and forth, with QB desktop. The "add myself as external accountant" is a function inside their Data file, which you cannot do unless you Own the Accountant Edition or are a paid Pro Advisor.

I think you might have mixed together QB Online and QB Desktop concepts.

If your client uses Desktop, you don't even need it to Remote Connect to their computer and drive it like a Zombie; but they cannot also be working on that same computer while you drive it like a zombie.

QB Online allows you to "be invited" as the External accountant, so that you don't use your Client's Log in to access their cloud-based data. That isn't the same as how Desktop works at all.