The SNES had a number of "enhancement chips" that were available in the cartridges. These chips did lots of different things. How did these chips actually transfer data to the SNES? How did the SNES use chips that weren't available at the time of its release?
The cartridge connector for the NES, SNES, and other consoles of that era directly exposed the main CPU's address and data busses to the cartridge, complete with the control signals you need to know what accesses the CPU is doing (reads, writes).
This allows cartridges to attach "enhancement chips" directly to the CPU as-if those chips were part of the SNES itself, in exactly the same way as the PPUs and SPC700 (S-SMP for audio) were connected to the CPU. A cartridge designer would know what hardware is in the base SNES (because that's fixed by Nintendo), and could arrange for their enhancement chips to appear in the main CPU address space as another co-processor, just as Nintendo arranged for the PPUs (for video output) and the SPC700 (for audio) to appear in the main address space.
Then, code running on the main CPU from the ROM in the cartridge would know where in the CPU's address space a given enhancement chip (in the same cartridge as the ROM) would appear, and can drive it by writing to the right parts of address space to send data to the enhancement chip, or by reading from the right locations to get results back for the CPU to make use of or transfer to another chip.
It doesn't matter that the chips were designed after the SNES was on sale - the cartridge designer is responsible for ensuring that the chips do the right thing from the CPU's point of view - and thus the enhancement chips were often designed around what the SNES expected.
How did these chips actually transfer data to the SNES?
How did the SNES use chips that weren't available at the time of its release?
Ever wondered how a PC can take advantage of a interface card that was not developed when the PC came out? Like a VGA card? Just by plugging it in, even before any DOS or extra drivers are loaded?
Because it brings basic drivers in an extension ROM. Plus drivers loaded by DOS and/or Windows.
It works the very same ways on a console. After all, the cartridge does not only brings some 'co-processor' but also a ROM with the game containing all code.