Development is done in a Ubuntu-Linux 16.04 LTS virtual machine. Ubuntu 10.04 and 12.04 are other development environments known to work for compiling the firmware, but might require some adjustments. See the Wiki for guides on how to set up a build environment.
Note that some components are proprietary to various entities (Asus, Broadcom, Quantenna, Paragon Software, Tuxera, etc...). For these, the precompiled binary blobs are included in the repo, and the build system is able to make use of them to generate a fully working firmware.
Copyrights belong to their respective owners.