If I were to guess, they aren't heavily modifying anything... it's likely a fork from Android and supporting React Native as a primary tool towards developers. I honestly doubt they've actually "done" much of anything other than branding and packaging.
Fire OS was an Android fork. Best I can tell from the stuff I've seen in the news about it, is that Vega OS is not Android at all, and is completely custom (well minus the Linux kernel)
