I use VocalPitchMonitor on android. It’s free and just gives me a dead simple line showing pitch. It’s not specialised for guitar at all, but all we need to know is which note the string makes when we pluck it

Tuner in F-droid is good. I’ve used several different android based tuner apps, and a couple Linux based tuner apps over the years and they’ve generally all been serviceable. Tuner is what I use at the moment.

I use ccgt on F-Droid.

I use insTuner on iOS. It’s a basic tuner, but seems to work well for all my instruments. It has a tone generator too, in case you prefer that method.

Spectroid or Voval Pitch Monitor on Android playstore:

https://play.google.com/store/apps/details?id=com.tadaoyamaoka.vocalpitchmonitor

I use Cleartune on iOS. Simple and accurate. Been using it for years.