There is a decent quantity of maps are open source so the .map is in the .zip with the .bsp.

For instance, Arcane Dimensions.

So you just open the .map, find the rock you liked and paste it in your map if your map editor supports that.

If the map isn't open source, you can just use a Quake map decompiler and then do the above.

Not much need for prefabs in Quake. 

There are no shortcuts in life! Take it from an old fart.

IMO copying and pasting from other's maps is not making maps. Unless you really know how to do it yourself that is. At this point you clearly do not.

Just learn how to make brush-based geometry yourself - the right way. By putting the work into it. I put my maps out there so people can learn from them, not copy and paste my hard work.

Now I've used fog settings and lights from people's maps but I usually tweak them or just redo them eventually. And it's always because I am not clear on something and want to learn. i.e mangles and delay/wait settings from Skacky or fog settings from Sock.

Not everyone feels this way clearly, but I sure as hell do. If you do copy and paste from someone's map PLEASE give them credit in your readme file.

But best to learn for yourself and that cannot be rushed. 

Some mods, such as Quoth and Arcane Dimensions, allow you to use custom models (.mdl) in your maps. They come with some, and you can grab additional models from other other places like Preach's blog:
https://tomeofpreach.wordpress.com/category/models/

Definitely give credit for anything you use in your map's readme.