Draw on a key position when the drawing is not relevant to a single frame rather than the video

A screen drwaing behaves as if it has been drawn on the computer's screen. It is independant of the video therefore when a drawing needs to be extended onto another video or will be used for one video after another (a template), then a screen drawing should be used.

The basic techniques and the most useful drawing tools are illustrated here

Every drawing tool with a black mark in the bottom corner is a group of tools. Reveal the other tools by right-clicking the button with the mouse

Spline tool is used to create curved lines easily with a mouse by clicking where the line will bend. Not only useful for curved shapes but also to track the movement of an object from one frame to the next as demonstrated here.