Step 1 was getting my hands on Raspbian. Step 2 was running OpenBSD on the Raspberry Pi 3 Model B. I had quite a few try & fails but it booted, installed and ran properly in the end. Full story follows.
Raspberry Pi can be used with a prepared OS, booted up and just used. But when it comes to customising or doing something else than what was planned, you may need to use the console. And that requires a TTL adapter connected to the GPIO. The thing is, using MacOS is not as straightforward as I thought. So here are my notes.