Someone on A.net will always get snarky about knowing the answer. It's just how things happen here (for some reason).
As for choice seats, if you have preferred status with US then you can select them at booking. As a preferred member I have never had trouble getting one.
If not preferred, you can pay for them, or hope to snag one on check-in (though I think you need to see an agent to try for one without paying).
The seats are generally in the front of the Y cabin, exit rows, and bulkhead seats toward the back on select aircraft with mid-cabin equipment (757).
I want to live in an old and converted 727...