How “exactly” does the MIK system work?

I bought a bike with a MIK rear holder (excuse my missing words in english).
I know that Basil (and a bit Giants) make some official stuff for their MIK system.
But honestly, their bags look so boring.

How exactly does the MIK system now work? Can I get attachments to just get any bike bag?
Is there any consideration because of "safety"? When I rode those bikes in the netherlands, I was not worried about people stealing my stuff from the bags. In my homeland I am a bit worried, how can I secure it?

What other recommendation do you guys have for accessories?

My Bike: https://www.tenways.com/de/products/ago-air