Where can I Learn how to work with the tiny cores?
"Work with"? As in, develop an application intended to run on an ATtiny processor? As in, port an Arduino library to an ATtiny processor? As in, add support for an ATtiny processor not yet supported?
I've never written code for an 8 pin chip.
It's not much different then working with ATmega processors. Some of the differences...
• Less memory (obviously) (a few have no SRAM)
• Fewer pins (obviously)
• Fewer timers (one or two instead of three or more)
• Odd-ball timers (Atmel seems to be targeting specific applications / markets with the ATtiny processors; they created more idealized timers for those applications / markets)
• Very few have a USART
• None (few) have dedicated SPI or I2
C hardware (they have a "USI" instead)
• I don't think any of them support an asynchronous timer
• Some of them have an ADC with more features
• Some have no ADC
The ATtiny84 processor is a great place to start. It's like the engine in my old Jeepster Commando: an eight cylinder engine with two cylinders chopped off. It's very similar to an ATmega328 just without timer 3.
I would like to learn what or how much of Arduino C and C++ works and where.
Everything C and C++ works. You will have problems with libraries and low-level code. Be prepared to read the datasheet.
In my experience, the best place to start is with a specific project. Get some guidance on which processors would be good choices and then go to work.