Mobile doodling with finger, and being able to scroll?

I am building a doodling app and I would like the user to be able to scroll the page and doodling with their finger.

I presume I must use buttons for the user to be able to switch between doodling and scrolling, however, I would be interested to know if there are any other solutions that may be better.

a. My solution was to simply have the pen and eraser tool be available to use, but by default the scrolling would be the primary interaction. Which is not ideal as I would like the user to interact with the page first by doodling on the page (not scrolling).

b. I run into the issue that if the user has pen tool (doodling) as their primary interaction, they won't realize that there is a way to scroll as well (unless they tap on the pen tool to clear the selection)

Any thoughts on this.. Is there a better solution?

Thank you.