I have been looking and looking to find an explanation of how to add text to a video - and to decide form when and till when the text shall be shown.
Does such an explanation really not exist?
Text is added via a Title. There are several ways to invoke that function. Once can to to the Toolbar and choose Title>New Title, or from the Title "showcase," can choose to use, and modify a Title Template.
To have a Title, with Text, or even Images and Clip Art, above (superimposing) a Video Clip, just create/modify the Title, and place it above the desired Video Clip.
As PrE assumes that one wishes to create and place a Title, at the location of the CTI (Current Time Indicator/Playhead), I like to move the CTI to the end of the Timeline, create, and then modify the Title, and click+drag it to the desired location.
The modification of Title Templates, or the creation of a Title from scratch, is done in the Titler, a sub-application invoked from within PrE.
How to create and use a title also varies a bit from version to version. So, if you want specific instructions, you'll need to tell us which version of the program you're using.
Meantime, my book offers step by step instructions for every tool in the program, if you're interested.