When I display the PKToolPicker in my app it looks similar to the image below
If you use one of the apps from Apple like Pages or Freeform you see a picker similar to this one.
Notice that the PKToolPicker from the Apple app has 6 tools not including the ruler. With the tool all the way to the left being the writing tool.
How do you get the picker to display with the writing tool? I have looked over the API and I can see where you can set if the ruler is displayed or not but I can't find anything for the writing tool
Good news! Adding the Scribble item to PKToolPicker is now supported in the beta of iPadOS 18.
Check out the documentation and related WWDC video for more details:
https://developer.apple.com/documentation/pencilkit/pktoolpickerscribbleitem