how can you draw inside a shape in preview app?
Remove the fill of the object (i.e. select the diagonal line to the left of the black and white colour line - see image below) but leaving an outline, draw inside, or on top of it, then add the fill again.
 
If it's a complex fill, save it first, by clicking &quot;Show Colours...&quot; and adding it there.
Note: this will not group the two layers drawn and they will have to be moved individually, or by selecting them both (with the shift key).  If grouping is expected, I refer to @benwiggy's comment to use a dedicated drawing application.
PS. If flattening the image is of no consequence, consider theonlygusti's comment, or the linked to previous answer to this question.