It is about making elements grow in AE and its the same way I write on text.
You basically trace your text using MOTION SKETCH then apply those keframes to the brush
position of the WRITE ON effect then use that layer as a LUMA MATTE so that the text writes on
exactly how the font looks.
You want to draw a line with the pen tool and use “stroke” instead of vector paint. Then you can animate the start and end to control the length of the stroke.