measure magnetic field

[TimTDP]

I wish to install a sensor with internal GPS Receiver and Compass to my boat
I am worried that magnetic fields, caused my vhf radio / antennae; boat electronics, battery charger, may effect the compass
I have a Samsung S21 smartphone.

How would I use the phone and Phyphox to monitor the magnetic fields? Sorry, I am not technically minded!

What is an acceptable magnetic field level to prevent compass deviation?

This is the sensor:
AS GPS HS - External GPS Receiver with Heading Sensor | Humminbird (johnsonoutdoors.com)

Thanks in advance