This is still somewhat confusing as it has you editing files that may not even exist and aren't accessible by the default samba shares. Also, leaving out that chmod step was very bad. I may need to write my own better guide if nobody else has made a simple one.
Anyway, I did this over ssh to make es_systems.cfg accessible so I could open it in Notepad++ on Windows
sudo cp /etc/emulationstation/es_systems.cfg /opt/retropie/configs/all/emulationstation/
I also had to do
sudo chmod -R 0777 /opt/retropie/configs/all/emulationstation/es_systems.cfg
in order to allow me to edit it. This is not easy stuff for dum-dums. It needs a script really. People who just want to play Pico-8 games cannot understand all this arcane wizardry.
Also had to switch to using pico8_dyn
I shouldn't have to choose between running the pico8 cartridge directly versus running splore. I should be able to do either one, anytime. How do I fix this?
(later edit) More recent guide here: https://retropie.org.uk/forum/topic/30736/how-to-install-pico-8-on-a-raspberry-pi-3-or-4-for-dummies-2021-version?_=1627524066426
