These are individual files from the chosen icon-Theme. Of course you can use them for any purpose.
Where they live depends on the chosen icon-theme.
This is an Icon of the Theme "AwOken" in the size 128x128 pixel. The application-Icon are found in the subfoilder "apps" of each theme.
If you have installed an individual icon-theme (not root) the icons are saved in your home-directory (hidden sub-folder ".icons").
An icon theme can point to another icon theme for all the icons it doesn't support itself. Therefor each icon them has an initial-file called "icon.theme". The row starting with "Inherits" points to the other icons-themes, that should be used in case of a missing icon.
Comment=Smooth modern theme designed to be intuitive.
Theoreticaly, you can build your own icon-theme containing only one icon and including another icon-theme for the whole rest.
I hope this answers your question. Please excuse my poor english...