Are invisible sprites loaded into memory? [answered]

Do they use the amount of memory a visoble sprite would use or is it reduced to just the position and instance variables ? And what about the layout parts which are completely empty, but still in the game - How do they affect the memory?

Pretty sure all sprites (visible, invisible, off-screen, with all their animations) are loaded into memory on start of the layout.

You can easily test it - put several big sprites on the layout, make them invisible, run the project in debug mode (Ctrl-F4) and check memory usage.

All objects that are placed on current layout - that is currently running, are loaded into a memory. Invisible Object placed on layout are simply not rendering. And if you don't know, there is a difference between changing objects opacity and changing objects visibility. You can refer to the manual for more information.