How Do I Make Rocks N' Stuff Like In Badlands Maps?

I'm assuming you're wanting to add something called displacements to your map. Both Sandwhip and Freyja have good guides on them:
Sandwhip: https://tf2maps.net/threads/a-bumpy-ride-a-guide-to-displacements.42147/
Freyja: https://tf2maps.net/threads/guide-displacements-basic-construction-part-1.24966/

Don't worry if you're not very good at them. I've been here for years and I'm still not confident with them.

If you're not talking about displacements and are instead wanting the rocks that jut out of the ground like this: You'll be needing to use a prop static.



The way you add one of them is by placing an entity down with this tool:

You then go to the class, type "prop_static" if it isn't there already.
At which point you click on "World Model" and then "Browse".
If you then search for "Rock" or "Rock00", you'll be given a list of rocks to choose from. Click on one, then hit enter. And then in the object properties hit apply. And you will have your rock prop in your map.

I've seen more that are called boulder on the model browser. or use the ABS resource pack with the custom map file that lists every prop before JI Update