Simple question, hope I can actually get an answer. Here goes. I chose to install ALL of my games in a dedicated folder on a dedicated drive. However I noticed GOG Galaxy installed parts of itself in other places on my machine...WHY? I didn't ask it to, I never specified it could do so. I am more than your average power user so I don't like software not doing as it is instructed. WHY DOES THIS INSTALL TO AREAS OF MY HARD DRIVE OTHER THAN WHERE I SPECIFIED?

Please, seriously, help me solve this, or conversely fix your code so it installs where it is told to. I never gave it permission to split it's install and/or store data ANYWHERE ELSE ON MY MACHINE.

Galaxy installs itself (by default) to Program Files (x86) and it stores its preferences to %ALLUSERSPROFILE%\GOG.com
It also has 3 folders it uses for the stuff it downloads, which can be configured. Those are the location to install the games, the temporary folder where it will be storing the stuff it's currently downloading, and the folder where the extras will be moved once downloaded. Those options are stored in the registry, though quite a few people have had trouble saving them, due to a registry permissions issue.

Assuming you configure Galaxy correctly, the only folder stuff will go to without you explicitly telling it to store in will be the %ALLUSERSPROFILE% folder, which is the recommended folder for programs that are installed for all users on a machine.

There are some places on the system that are designed by the OS developers to be used by applications installed by user. Like the program data folder on Windows or Application Support on macOS.

App Developers sometimes cannot put everything into one place because the system forbids them (due to for example security).
Example: to register a daemon that runs on macOS with root privileges you need to put a plist configuration file into specific system folder, otherwise it won't work.

So by saying "put everything where I want" you just proved that you're not so power user as you think ;) (No offence).