How do I delete my account on the iPhone

Perform this procedure:--> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

This process erases all your data on the device and the device itself is as if it were brand new.

Regards

Giulio

1 - First, sign out of Find My iPhone here:

Settings → Apple ID → Find My → Find My iPhone → disable Find My iPhone

2 - If you've switched to Android, turn off iMessages by going to:

Settings → Messages → deactivate iMessages

If you are still using iPhone, then you do NOT need this step.

3 - Then, go to Settings → Apple ID, scroll to the bottom and select Sign Out.

Now your Apple ID is removed from your iPhone 8.

4 - Now you can restore it to factory settings so everything will be deleted. Go to:

Settings → General → Transfer or Reset iPhone → Erase All Content and Settings

The phone will restore to factory settings and boot to the "Hello" setup screen. After that, everything will be gone from it, and it will in principle be ready to be sold or passed on.

You can find more useful information here:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support