hi, i was wondering how i could make an object like a cave in the sid of a mountain. i noticed that some of the stock props have purple polyhedrons that cut into the terrain and/or other props, can someone show me how to replicate the effect?

It's called a "terraincut". I know Fragme! released some seperate terraincutters somewhere on this forum. You could try looking at them to figure out how to replicate the effect.
EDIT:
Here is the link!

...or follow this tutorial.

Or that... Yeah, I just remembered the terraincuts because someone had bumped the topic a long time ago, so I thought I'd help to the best I could