Edge of world ideas

so in making an “open world” RPG I came across something I am unable to figure out and it could be that I am just not wording it correctly. In most games that I’ve come across that are open world there are things at the edge of the map like ledges, cliffs, static buildings, etc. If you come to a cliff where if you fall off you die, there are buildings, scenery and things of in the distance. I’m just curious as to how this type of artwork is done. I’m a noob at this and basically doing it for fun and learning. I know things are way advanced when you start talking about big time publishers and game designers like Bungie(Destiny) so ships and things like that moving across the sky I’m not looking for. I’m thinking more stationary artwork, and how to accomplish this.

Either an image on a plane or just very simple 3D geometry.

Some games do a “Get back to there or you’ll be shot” thing where you die like within 10 seconds after going off the allowed area. Some other games do it like you are blocked by natural or man made elements i.e fallen trees or walls. Are just looking for ideas?