1) When you create a Text annotation with a single click, the focus goes to the text to edit in the box (although the blinking vertical line is really difficult to see). But if you click and hold and then expand, Zotero considers the click release as an actual click, thus loosing focus from the text box. For example, if you release on another annotation, that annotation will get selected, cancelling the text annotation you were trying to do. Even if you stop at a random place without anything, you will still loose the focus of the text box, so you need to select it again manually to write in there.
I was expecting the click and hold to allow creating text boxes of different sizes, but that is probably secondary.

2) I can select, highlight and underline some text behind an annotation by pressing Shift on the keyboard. I can also make Draw annotations anywhere I want without the need to press Shift. But I could not find how to start the other annotation tools on another existing annotation or on hyperlinks of the PDF file (Select Area, Add Text, Add Note)?
For example, if I click on the Add Text button, obviously I want to use it anywhere I click, even if it is on existing annotation. But if I click on an area annotation (which does not have comment text yet and never had any comment), the focus will jump to the comment of that area annotation, and the text that I will type will go there. If the area annotation already has a comment on it, the click will simply select the area annotation. The Add Text button will still remain toggled though, so I will still be able to make the text annotation somewhere else without pre-existing annotations.
I guess it would make more sense for these annotations to behave as Draw annotations, which means that it does not care about what is below when you do it.

3) All annotation types stay activated after finishing the first one, so that you can make multiple of them in a row, except the Add Text and Add Note which can only do one and then get un-selected. So if you want to write text in many places in your PDF file, you need to click on that button every time. Is there a reason not to keep the button toggled even after finishing writing text in them?

4) Bonus: The nice trick of pressing Shift to access the text behind an annotation does not work yet in the EPUB and HTML viewers.

Zotero 7.0.0-beta.36+3ef1f0992 (64-bit)
Windows 10