I only have 1 tuning fork but I never use it! 🤣

I tune aurally by listening to the beating of the intervals and making adjustments so they fit together in what is called a temperament.

I use an ETD (electronic tuning device) for setting my starting pitch A4 (usually at 440Hz), but some use it for the whole piano. You need a specific piano tuning one that recognizes inharmonicity; a guitar tuning app won’t work quite right.