What /u/[email protected] mentioned - that’s correct. But to elaborate: The icon image isn’t simply stored as an image, but kinda complex (out of scope to explain the whole concept, but you can read more about it here

What windows does the first time it comes across a new .exe, is extract the image, because this is an “expensive” process, it’s optimized to not do this every time, and instead the extracted image is dumped into C:\Users\%username%\AppData\Local\Microsoft\Windows\Explorer in one of the iconcache.db files.

When you recompile an exe with the same name but will a new icon, it will actually have a new icon, it’s just not shown, because the old icon is retrieved from the iconcache.db. You can get around this by deleting all your iconcache.db files

I also need something in case I make everything “from scratch”, like just compiling something from the command line.

Also, even if you compile from the console, you can usually still specify an icon on compile time, and don’t really have to inject it later after compiling. But of course that depends on the language / compiler on how you’d have to do that