Hello!
Puzzle Spoilers below ---------
The door location is near the top of the cliff, but it's hidden behind a lot of dirt. You can... clean it away! Or move, kick, or a few other verbs to reveal the door, I believe.
You can currently use the flag, but it's not too important for now. You will likely know what to do with the flag by progressing further!
Also, yes, the monolith is solved upon seeing the fire. This will also make sense upon progressing a little further.
Sorry for the several-week late reply!

