The board can decode any I/O port and of course RAM/ROM cycles. As it use a Pico2, its RAM is doubled vs Pico1 and it can access a 8MB PSRAM as well.
No 5V pin go to the pico. We use some 1 buffer, 1 level shifter, 3 multiplexers. We write to the CPC with 3.3V, we use 5V to the CPC only to drive some signal like A15 for C3 memory mode and IOReq to disable intern floppy.
The PicoCPC does NOT use a CH376 chip, everything is done with the Pico itself. PicoCPC handles FAT12, 16, 32, ExtFS. CH376 will be emulated on the board, with nothing to do at the CPC side.