Fritzing app comes with a various built-in parts. But in reality the number of different modules/boards/electronic components/etc is so vast, so it's really hard to cover all of them within a some specific team, therefore, there is a possibility for adding custom parts. The official guide for the part file format can be found here.
This repo contains a set of manually built parts for the Fritzing app. The Inkscape app is primarily used for drawing SVGs for different views of each particular part.
There are other resources where custom Fritzing parts can be found:
- the official Fritzing forum where people around the world submit their customly built parts
- Adafruit Fritzing library
Links to the particular custom parts I use frequently or at least have them: