A bluetooth and USB-C modification kit for the iPod Classic: https://moonlit.market/
I left my home country for Portugal when I was 18, and was refurbishing iPods for money while living in a hostel. In my free time, I developed a Bluetooth kit, a crude first version which required hours of hand-soldering soldering, CNC milling, etc. to manufacture. While selling these I used the money to prototype and manufacture the second revision.
That's the version you can purchase today, which can now be assembled in about five minutes. This has allowed me to scale the business, and now we're a few employees big, shipping worldwide. A few grey hairs have appeared at 22 but finally I can relax a little.