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
how ?
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.