Do I need to buy a hose separately?

Yes. This is the adapter and banjo bolt only. You'll need to source your own 4AN braided oil line, cut to length, and route it from this adapter to your turbo, supercharger, or cooler inlet.

Where does the banjo bolt mount?

Banjo bolt mounting points vary by engine block and turbo kit design. Verify your block or adapter has a threaded hole before ordering, or test-fit to confirm thread compatibility.

Can I reuse this if I swap turbos later?

As long as your new turbo kit uses a 4AN inlet and your block has the same banjo bolt hole, yes. The adapter itself is reusable; only the hose line would need replacement if routing changes.