It's essentially a copy of the PhotoPlug that doesn't cost €35, it plugs into your phone's 3.5mm jack and masquerades as a microphone while using an app called Shutter-Speed. I'll show you how to make it for less than $10:

What you'll need:
BPX38: https://a.aliexpress.com/_ooNVS3N
3.5mm male 4-pole (TRRS): https://a.aliexpress.com/_okCBOkR
4.7k ohm resistor: https://a.aliexpress.com/_oCb7WGP

Instructions:
Step 1: Take the 3.5mm plug and bridge the sleeve (mic) and ring (ground) next to it with your 4.7k resistor.
Step 2: Your phototransistor has an emitter, base, and collector lead. Snip off the base.
Step 3: Solder the BPX38 in parallel to the resistor. Make sure the emitter is connected to the ring, and that the collector is connected to the sleeve.
Step 4: Cut off the cable strain reliever and screw the plastic cover back on.

There ya go, enjoy not having to worry about exposure guesswork with your mechanical film cameras anymore.