My editing screen looks a bit different to yours, so not sure if it’s a difference in version (I’ve got Gallery version 12.01.07.5 on my UK S10 on the latest One UI 3.1 here); device setup; country/network variation; or if I’ve not taken the right steps to replicate this one. 😕
I went into Gallery, clicked on a video, clicked on the pencil icon (the edit tool), and then the 'T' icon (text tool):
Clicking out of the keyboard brought me to:
The text in the box can be edited by clicking on it; and the box can be moved around, resized, or deleted using the corners of the box; and the time it appears in the video can be changed using that slider below.
But I found that once I'd saved the edited video, the text and box can't be edited or removed (although, you can then add another text/text box to that edited video if you want).
I've been in the " Create movie " layout, so that's why it looks different. Could you pls check there? I have the same Gallery version.