Linux distributions are a different beast since they're mostly tracking a myriad of upstream third-party repositories overlaid when necessary by their own patch

Linux distributions are a different beast since they're mostly tracking a myriad of upstream third-party repositories overlaid when necessary by their own patch sets. And most of those upstream repos are by necessity highly decoupled from any particular distribution. Different distributions have different solutions to how they manage upstreams with local changes. E.g. OpenEmbedded uses BitBake where (similar to FreeBSD Ports and Gentoo Portage) the upstream can be pretty much anything, including just a tarball over HTTP, and local changes are captured in patch files, while others instead use one-to-one tracking repos where local changes are represented by version control revisions. AOSP is closer to what you're imagining, but I haven't met anyone who thinks Repo (Android's meta-repo layer) and Gerrit (their Repo-aware code review and merge queue tool) are pleasant to work with. E.g. it takes forever and a day to do a Repo sync on a fresh machine. A demand-synced VFS would be very nice for AOSP development, even though it's not a monorepo but a polyrepo where Repo ties everything together.