Let me give my best shot.
I believe everything is drawn manually from the beginning, she has her style because of custom brush, let's say she is using Photoshop.
What you can is following:
- If you have brush around you, make a stroke with traditional brush
- Do it until you find any good texture or anything you want to achieve
- Scan this brush stroke (with 600 DPI or 1200 DPI is even better with million of colors, if your scanner have "sharpen" option, drag this to "high")
- Define it as brush in PSD
OK now you have your own brush set, you can draw shape with pen as the base or you can freehand it from the beginning, then you just need to apply that brush along you need it to create that texture feel,
- Make a basic shape
- Duplicate it and use another color (either shading or lighting)
- try to erase some part with the custom brush