The size of the mockup displayed on the screen is almost full-scale iPhone SE. You can do prototyping with imagining of the actual screen display. Since the exported image is real resolution of iPhone SE, you can use that as it is in other many prototyping tools.
There are tools that thin pen and thick pen for drawing. Thin pen is suitable to draw the UI component. Thick pen is ideal for marking and coating. You can choose color in 5 colors, those are just right amount. You can use colors by the role for example“Blue is for button, yellow is for emphasized.” etc., whatever you prefer.
There is also a stencil function that can draw the common icons on any position. It will make your prototyping work faster.