I need a whole Arduino data center to blink an led.

You could just read the documentation to find the answers.

That's pretty sweet. Do they expose an API or are you reading from memory addresses?

There is a small part of me that likes the little puzzles, but that's largely overwhelmed by resentment.

I agree with the sentiment, but that's a clip from a 2016 video. youtu.be/uigxSZE7uwI

Arduinos have exposed pins and bootloaders designed for modifying firmware. While there are many cases of commercial products exposing serial pins, it's a wild leap to suggest any traffic light has the same vulnerabilities.

Come eat at the trough, piggies!

There are a number of ways. To list a few:
- ESPHome
- MQTT
- HA REST API