can someone tell what mod it is that turns the guy into a girl?
Not a mod. That's a feature of the
core SDT game.
The first tab of the in-game menu is "Scene". You can reach it by pressing the Y shortcut key.
Within the Scene tab, the rightmost pane contains customization options for the standing character (who is typically male).
You can scroll down within this pane by using the elevator bar. Midway through the list you'll find a selector labeled "Body". You can shift it from Male to Female.
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There are some mods which create
different male and female bodies. If you're interested in one of those then please post a screenshot (or describe it in greater detail) so that we can point you in the right direction.