Pico should be usable by default on onionos just needs to be enabled.what version of onionOS are you running?
If you navigate to Apps>Package Manager>Expert there should be something in there to enable pico8
Does package manager even show?
Edit: here's the steps for onionOS provided you have the most up-to-date firmware
Turn on your Miyoo OnionOS device and navigate to Apps>Package Manager>Expert.
Scroll down in the system options until you see Pico-8 Standalone. Enable it.
Take the pico8_dyn and pico8.dat files from the licensed download package and place them into the /Rapp/PICO-8/bin directory on your OnionOS SD card.
Reinsert the SD card into the device and select Run Pico-8 w/ Splore under the apps settings section.