The place to start is the Gimp docs as previous post. Then just dive in and experiment.
The main thing about the Unified Transform (UT) tool is combining multiple transforms in a single operation minimises intermediate interpolation you get using separate tools. You still get interpolation which can degrade an image but it is a s small as possible depending on the image.
1. Click and drag inside UT to move. Click and drag outside UT to rotate. You can do these any time between other transforms, no need to select another tool.
2. The documents show the various handles for resize ..etc. There is an icon next to the cursor when over the correct handle. Make note of that, easy to rotate when you mean to scale.
3. Try out the CTRL and SHIFT modifiers and see what they do, especially with the resize (square) handles. CTRL imposes overall symmetry. SHIFT in one direction. SHIFT-CONTROL symmetry in one direction.
Not a tutorial, just a quick, 50 second animation,
https://i.imgur.com/BUP0Wfw.mp4 Still the best way... practise.