Amazon (today) maintains a git repo per package, though in theory there's no reason why it has to stay git (or even be homogenous). Each application is a set of versioned artifacts that can be updated, forked, or merged together so long as the whole set builds together.
I've never understood the mono repo argument. With something like Brazil (or even sonatype nexus), you end up in a much stronger place for guaranteeing consistency and immutability in your builds.
