Here's the things that helped me to do the things for my Miyoo Mini+ and Onion
In short, format an SD card to FAT32 (not exFAT) and unzip the latest Onion to the root of it
And it woudn't hurt to buy a faster/better micro-SD than the stock. Spend $10 bucks
Check this collection of archive.org links: pinboard.in/u:shmup/t:romsets
Roms must go in specific folders as seen in this Quick reference
Check the FAQ for specific questions
Learn how to netplay with or without wi-fi: docs/multiplayer.html
Here's the netplay cores config: netplay_cores.conf
Check out #miyoo-mini-plus on Discord
Or start start a discussion on Github: github.com/OnionUI/Onion/discussions