You 3D print the cartridge and you buy the hardware.
These come straight off the card with no grading at all. The cartridge records at 1296 × 972 and 17 frames per second, which is the rate this particular camera's motor actually turns at.
Are you that person that likes watching a diagram instead? Here you go. The diagram below is live, so you can try the whole thing without soldering anything.
This one pin feeds both halves of the trigger circuit, running the LED through a 330 Ω resistor and the LDR through a 10 kΩ divider.
The Lipo Rider Plus does two jobs at once here: it charges the cell over USB-C, and it boosts that 3.7 V up to the clean 5 V the Pi needs to run.
For the Pi to read this reliably the voltage has to climb past 1.8 V when the light gets through and fall below 0.8 V when it doesn't.
It all comes down to just 7 easy steps to build the cartridge. The only long one is the install, and even that is mostly just waiting for the Pi to finish pulling down packages.
You get the model, the print profile, the full wiring schematic and all of the code that runs on the Pi. It is free and open source.