I'm interested in writing an "Arduino core" (i. e. porting Arduino to a new MCU and new board(s)). I'm having trouble finding much documentation on how to do so. Am I missing something?

This page says "anyone may develop a core for their own boards following the rules and requirements we have issued." So, where are those "rules and requirements?"

I found something called the "Platform Specification", which sounded promising, but it only documents the config files; it does not say anything about the code that needs to be written to support a new platform.

I found ArduinoCore-API, which says it is the "hardware independent layer of Arduino core." But it doesn't mention anything about how to write the hardware-dependent part of the core.

I'd be grateful if anyone could point me to the Arduino core documentation. It's not turning up in my web searches. Thanks!