is it possible to open an app on my phone, when I use nfc in home assistant app. ?
You can use the intents to do stuff in the phone.
But there is no guarantee that what you want will work.
Edit: this post is based on Android phones
my phone is android
Look at the android intents topic here, and documentation of HA app.
There is no easy answer, you have to try and see what works.
Link to the docs