Where is the grounding in this fixture box
There are 3 twisted together copper wires up there that you can see in
the photo.

Yes the bare copper wires are the ground wires.
You will connect the ground from your fixture to them. It appears there at least 3 of them so you will need a larger wire nut, a red one for 3 wires or a green one for 4 wires. (Wire nuts are color coded by size, you will probably need the green)

Or is the fixture itself grounded?

Fixtures are only grounded when they are connected to the grounding of the electrical system of the building.
Twist on wire connectors Wiki.
Per Ed Beal's comment:
It difficult to see if the box is connected to the ground wires. If not you can use the Greenie, it a green wire nuts with a hole in it for a single wire. That single wire can then be attached to a screw on the box. You can find YouTube videos on how to use them.
If a metal box is grounded as it should be then depending on how the fixture is set up it will determine how the ground connection is completed. You may need to attach the ground wire from the fixture to a screw in the box or it may have its ground wire connected to its mounting bar and that bar will be touching the metal box, thereby making the ground connection.