The Externals are one of those concepts from the 90's that was never really fleshed out. As for the 'generations', it doesn't really work like that either, as in NM 1, Hickman specified six generations; the ancients(which would include all of the Externals), the elders(not sure what the cut off point is, but I assume this covers the Victorians like Mystique/Sabes/Logan up to Magneto and Xavier), the 05, the other X-Men, the school squads, and then the modern Krakoans. So, maybe 'first of his generation' for A refers to, let's say, the 'historical' era(Sumer and KMT onwards; recorded civilization). Selene is from the Hyborian Age, something like 17,000 years ago, whereas En Sabah Nur is from about 5,000 years ago.