Sadly, since most smartphone magnetometers seem to have a sample rate of 100/s, this will not be applicable to Americans and everyone else with a 60 Hz grid frequency, the 50 Hz were already at the Nyquist–Shannon limit.
In Germany phyphox is quite popular in physics education.
However on android the sampling rate of the acceleration sensor is limited to 50/s. At least if you install through the official app store.
[1] https://iopscience.iop.org/article/10.1088/1361-6552/aac05e
My understanding is that it’s the same even on iOS (or at least on my iPhone SE 2020). More specifically, the output only measures till 50hz (but the sensor sampling rate is actually 100hz - Nquist, you need double the measured frequency as sampling frequency, yada yada.)
My parents have a sound bowl, and I wanted to know the resonance frequency. Took an audio spectrum, zoomed in on the first peak, read the frequency (iirc it was around 208 Hz).
The interface is more polished, but the information is less technical than Phyphox (as the app is geared towards being a survival toolkit).
Most recently I used it to check light levels at home in different rooms, to determine where we need to boost or replace LED strips. Sure, there's million Lux meter apps, but Phyphox is better than all of them and demonstrates why these things shouldn't be dedicated apps in the first place. In the past I also made use of EM and vibration frequency displays to troubleshoot hardware.
A complement to that is https://play.google.com/store/apps/details?id=org.intoorbit.... which, once upon a time, helped me track down a source of rage-inducing, late-night high-frequency beeping that was driving us insane - down to specific apartment in a block on the other side of the street. I ended up friends with those neighbors, after teaching them how to disable the alarm clock on their Bluetooth radio when they go away for a weekend.