On the TI-84pce you can send and receive files, appvars, etc. through a physical connector cable. Is there a function or way to use that capability through the ce-toolchain?

I want to be able to send/receive data to another connected calculator almost like a TCP stream while running graphics etc. in my program. What is the protocol that the calculator uses for this? I couldn't find anything on this on the ce-toolchain documentation. Do I have to manually edit memory with my c code to achieve this? How low-level do I have to go? Is this even possible to begin with?

There are libraries under development for using USB on the CE, but they aren't released yet. I'd recommend checking out either the usbdrvce or srldrvce-rewrite branches of the toolchain if you're impatient, or if you want to wait they will probably be released fairly soon.

The calculator uses a custom protocol for sending variables, which really only makes sense for sending variables. While you certainly could use the same protocol that the OS does, it would be better to just make your own protocol on top of either usbdrvce (if you only want to communicate with other calculators) or srldrvce (if you also want to be able to communicate with computers, or other devices like Arduinos).

What's the difference between usbdrvce and srldrvce? If I only want to write code for when two ti84pce calculators are connected, which branch should I use? Also, how would I test this? Is the only way to just get two ti84s and resend every time?

usbdrvce is a library for accessing USB directly, while srldrvce sits on top of usbdrvce and makes the calculator pretend to be a serial device, which is sometimes useful and sometimes not. If you only care about calc-to-calc communication, you should probably use usbdrvce with link_library. There's a branch of CEmu that supports USB host mode which you could use for testing. But for the other (device mode) calculator, there's not much getting around resending a program constantly while testing.