What are the methods an application can use to auto-start (in kubuntu 18.04)?

I know of System Settings/Startup and Shutdown, and I've found ~/.config/autostart and ~/.config/autostart-scripts. But what other methods can an application employ to auto-start on boot/login?

This question is prompted by accidentally running KOrganizer which then auto-started every time I logged in. I solved this somewhat brutally by removing KOrganizer but it has left me wondering what might be auto-starting in the background without my knowledge, so I would like to be able to check.