[c_o_n_v_e_x]

>They are a more accurate version of an IMU to use for dead reckoning. They will still drift over time.

A magnetometer is a completely different device than an IMU.

A sensitive enough magnetometer coupled with accurate magnetic maps can act as a GNSS alternative, albeit not as spatially accurate. There's a handful of companies currently working on quantum PNT and magnetic/gravity map matching solutions. I work for one of them.