...how are they jumping from system to system? Are there mass relays or not? If there are, well, that's auspicious, but if there aren't, how do they manage to go from system to system?
Screw that, how in the blue blazes did they get to
frakking Andromeda galaxy?
Mass relays were Reaper (or Leviathan?) tech, and they always seemed like being endemic to our own Milky Way galaxy. So, if they jumped to Andromeda via some super special mass relay - which they somehow managed to connect to, despite all (?) pre-existing relays having been wiped out by space magic, that would imply that Reapers, or the Leviathans, were much more widespread than we've previously thought.
If they didn't use a relay, then that implies that the galactic civilization has produced an effective very long range method of FTL travel, and if that is the case then they really don't need relays to move from system to system within Andromeda galaxy.
All things considered I would put my money on the latter case. After the cataclysmic ending of ME3, one of the most important things for getting the galaxy on its feet again would have been establishing reliable transportation without the relays. It seems to me as though it was always possible to travel from system to system in FTL without using relays, but since the relays
were there, no one ever had the drive to develop those systems further than what was needed for effectively maneuvering within systems - from planets to relays and so forth.
Odds are that this super-FTL will use quite a bit more energy than the FTL systems used before the relay system collapsed, and considering the way resources have been mentioned, I would hazard a guess that this might be related.