Tap on the zoom tool located at the lower left corner of the page, under the Write mode, to zoom in to your notes.
Alternatively, you can also use 2 fingers to pinch and expand the page to zoom-in.
To move manually to the next empty space, you can either move the zoom active window or just tap on the left and right arrow tools as indicated in the screenshot below:
To move to the next line, move the zoom active window manually or just tap on the enter button:
Zoom mode also allows you to automatically move to an empty space after you hit the darkened area on the right size of the zoom window, as indicated in the screenshot below:
A scrolling bar would be readily available on your iPhone screen, just swipe the right edge of the screen to scroll up and down your pages.