I always thought than in a memory-expanded Apple IIc fitted with firmware version 3 or 4 and an Apple-built memory expansion, the extra memory above and beyond the first 128K is connected in a way that's indentical to the way the so called "Slinky" Apple IIe memory expansion card for a II+/IIe works. I.e. there is no direct CPU access to the memory on the card; rather it uses DMA to transfer data between the 128K main/aux memory and the up to 1M expansion card memory.
However another question on this site stated that the Apple IIc uses the register $C073 for switching between several aux memory banks, like the Applied Engineering RamWorks.
So which is right - is Expansion Memory in a memory expanded IIc, using only Apple-Supplied hardware, addressed by the CPU like a "Slinky" card (write stuff to registers, wait for DMA to finish, see data from the card magically appear in main memory) or like a AE Ramworks (write stuff to register, instantly see aux memory go away and stuff from the card appear in its place)?