Most map apps assume you're always online: they stream tiles, hit a routing API, and quietly fail the moment you lose signal. Maps takes the opposite approach — draw a box around an area you care about, download it once from OpenStreetMap's Overpass API, and everything after that (browsing, searching, turn-by-turn directions) runs entirely from data already sitting on your device.
One request, then nothing
Downloading a region is capped at 50km² — enough for a city or a large neighborhood, small enough that the download stays fast and the offline dataset stays manageable on a phone. Past that one Overpass call, Maps makes no further network requests: no live traffic layer, no tile server to keep paying for, nothing to break when you're somewhere with no signal.
A router that isn't OSRM
Turn-by-turn directions inside a downloaded region come from a small routing engine built specifically for this app — not OSRM, not a hosted routing API, just a router that works against the same offline road data you already downloaded. That's the part that made "fully offline" actually true, rather than "offline except for directions."