Thanks for your response!
TreeItems are super flexible and are used for the editor Scene tree and FileSystem and most of the Inspector rows. Wherever there is a folddown caret, that's in a Tree.
It also creates the section dividers in the Inspector where you have the blank row with an icon in the center.
Your response made me dig a bit further and indeed there are some BoxContainers that are within some Inspector sections, which seems to be a VBox containing a Tree (or another VBox) for each section - so that answers the question about how TextEdit is inside the Inspector.