Was gonna use my PC for a bit while working on a project I started "CHIPping" away at... but I can't seem to find where the default carts directory is on the P-CHIP edition... where should I look?

The default cart location on the PCHIP is
/home/chip/.lexaloffle/pico-8/carts

It's a system directory and thus hidden from the normal file browser, so you'll have to get there via terminal.

I made a symlink to that folder from the home directory so I could easily get there both by typing and in the file browser.

From the ~ home directory in terminal:
ln -s NAMEYOUWANT /home/chip/.lexaloffle/pico-8/carts

Replace NAMEYOUWANT with the folder name you want to use. I made my folder "p8" so it was quick to type but you can call it whatever.