Yes, I have a compass built in. The LG Stylus 2 Plus was bought following a matrix of requirements. It is a HK version and it serves me better outside of US than in but then I decided my situation outside is more fragile and I need more help there . Indeed I can run an app but then I need the screen for Google maps and the tiny compass there can be easily missed and sometimes you enter a mode where it freezes.

The bottom of the phone seems to have particular impact on the compass, hence my attempt to push the latter forward. Regarding influences on sensors, the smaller the sensor the smaller the impact of nonmagnetized ferromagnetic material on the sensor. This is because the needle or some equivalent thereof sees the image of itself in the ferromagnetic material. Hence a small compass/sensor should be more immune to such influences, as the mirror image is tinier. On the other hand, ferromagnetic materials tend to be at some level magnetized affecting any sensor at the same level. In the end you see a combination of both effects, but smaller sensors, such as in a phone, have a chance to fare better. I'll try to do some tests comparing the phone compass with Suunto around the handlebars, but now have to work on things that were partly neglected when working on the compass mount