When you hit ‘start’ in the webui, it invokes java directly, e.g., java [add'l args] --jar spigot-1.212.jar, but within an instance of screen, which is a way to let the process be detached from the webui (so the game process and the webui process aren’t bound to eachother).
The webui only knows how to work with servers that are in named screen instances, though. So if you wanted to have a .sh script open up a webui-usable server, you should make sure the script starts the server via MineOS commands:
#!/bin/bash
cd /usr/games/minecraft
./mineos_console.js -s servername start