To be honest I'm not sure they work under the hood but they use debug mode and some other stuff to do what they do. Yes they can remove system apps, pretty much anything. They have a traffic light system to warn you about whether its safe to remove some things. Green is good, orange is a bit risky, red is riskier again. The risk usually relates to the impact on other apps or functions working.

Twas pretty easy to set up.

Edit: Screenshot addition. Not quite traffic light system but you get me. Unless they have illuminous pink traffic lights somewhere!