Or do the main developers for let's say the GIMP project just develop for all the different Linux distributions?
Thanks by the way
Basically, yes. It's not my place to answer for the developers, but here goes my understanding of it.
Almost all applications and components of the operating system are developer by their respective developers. Though specific applications are developed by the Linux Mint team themselves (which is being done in the open, see a full list here: https://github.com/linuxmint
Linux Mint, being a Linux distribution, picks the version of each piece of software they want to include for their users. They compile the software and offer the software packages (and source) for download on a central repository, and offer a "live session" CD or DVD image from which you can either test the Linux distribution or install it to your computer. Wikipedia can probably explain this better: https://en.wikipedia.org/wiki/Linux_distribution
As AlbertP shared, Linux Mint doesn't start from scratch on this. It comes in a main edition which is based on Ubuntu, and in a debian edition, which is based on (now act surprised
) Debian. So the Ubuntu and Debian Linux distributions are taken as a start by the development team (which means the majority of the software packages are already compiled and available in a repository). The Linux Mint development team generally adds software packages to the selection, or changes versions of selected software packages, to offer a better user experience.
So the Linux Mint team is writing part of the software of Linux Mint themselves, partly they are modifying software as developed by others (usually to fix problems with it in the Linux Mint distribution), and partly they are using the base provided by either Ubuntu or Debian. Finally, there is a testing team to do the actual testing of new releases of Linux Mint.