Finding a line with / does not select it - it only selects it.
To select a text area, click and drag on it or use the v , v or ctrl-v keys in normal mode (to select a normal area along lines or a rectangular area, respectively), then navigate using the normal navigation keys to expand the selection. This will bring you into visual selection mode.
When you select the selected text and are in visual selection mode, you can use the usual editing commands to change the selected text, for example. c will delete the text and put you in insert mode, y gU text, gU will change the text to uppercase, g? will be ROT13 etc.
As soon as you enter the editing command, you will return to normal mode and the visual highlight will disappear. You can re-select the same area with gv .
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