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this is what my search history looked like> gimp resize image
Image -> Scale Image
Very bizarre, indeed! Scaling the image is a "Scale Image" command in an "Image" menu.
> gimp rotate layer
Layer -> Transform -> Rotate { 90° Clockwise, 90° Counter-Clockwise, Arbitrary Rotation ...}
Totally undiscoverable. Rotating a layer is buried in the completely unrelated Layer menu and obfuscated by terminology. The user has to be a propeller-head who knows bizarre jargon, such as that a rotation is a kind of "transform" and might be hiding under a menu item labeled that way.
To add insult to injury, this menu item has not moved in at least 15 years to a more obvious place. Gimp clearly likes existing users better than new ones.
> gimp rescale interactive
In the latest stable Gimp (2.10.22) which I just installed last night, there is a surprise: I can't find the Rotate tool button. Aha, there is an "all in one" transformation tool. The Gimp version I previously used still had separate toolbar buttons for different transforms.
I see that we can right click on the all-in-one transformation button, and then pick a specific kind of transform. We want Scale, which is also available as the familiar Shift-S hotkey.
The Tool Options dialog comes up for the tool. I see that the very first item is a bunch of icon-labeled buttons next to a Transform: label. They all have tooltips: "layer", "selection", "path" and "image". Aha: Transform -> "image" must be what we want. We click on that.
Now we proceed to interactively rotate the image, and apply that change using the Scale button on the rotate dialog.
In summary: Shift-S to scale, pick "image", do the interactive scale, and click "Scale" to make it stick.
For your other queries, you just have to know how to work with transparent layers in Gimp. Gimp is not a carbon copy of Photoshop.
You were not born knowing how to erase pixels transparently in Photoshop.