> A device can triangulate its own location

Trilaterate (or multilaterate). Angulation uses angle, like a directional antenna, constructive/destructive interference for beamforming (this is how airplane landing systems work if I'm understanding it correctly), or optics like our two eyes, to find the angles to a target from known positions in order to determine its position in space

Trilateration is based on distances from known locations, determined either by signal delay (GNSS does that; newer cell towers also but call it "timing advance") or signal strength (used with both WiFis and cell towers)

> locally, given the WiFi hotspots

You'll also need a local database with the hotspots' positions (usually those aren't actually measured but estimated from observations at different locations). I'm not aware of a device that ships with this, nor popular software that uses it as its primary method, as such databases are many gigabytes. Thus this is typically not local; you're sharing your data (thus location) with the server which then kindly tells you where it thinks you are