Welcome to the world of Building Large Google Projects. This is somewhat painful in the Android and Dart/Flutter world. I haven't tried building Chromium yet but I'm sure it's just as complex.
Yeah, I've tried building Chromium on my own before. Chromium's build process is so inscrutable and changes so often that several Googlers I've talked to have said you can't really do it without handholding from another Googler in-the-know.
fetch --nohooks chromium && cd src
./build/install-build-deps.sh
gclient runhooks
gn gen out/Default
autoninja -C out/Default chrome
Congrats, you just built Chromium. Is it a different build process? Sure. But it sure isn't inscrutable, and they've been using this system for a while now.