A compound and a polish look the same in the bottle and go on the same way. The difference is how much paint they remove to get rid of a defect. Pick the wrong one and you either leave the swirls in or thin the clear coat for nothing.
What a compound does
A compound carries a coarse abrasive. It levels the clear coat down to the bottom of a scratch so the scratch disappears. That is what you want for sanding marks, deep swirls, bird dropping etches and paint that has never been corrected. It leaves its own haze behind, especially on dark colors, which is why a compound is almost always followed by a polish.
Rupes DA Coarse is the shop standard on a dual-action polisher. Rupes XC-2 is the heavier option for sanding marks and hard clear coats. 3D AAT 501 plays the same role on a rotary or DA.
What a polish does
A polish carries a fine abrasive. It does not remove deep defects; it removes the haze and micro-marring a compound leaves and brings the gloss up. On paint with only light swirls, a polish alone may be all the correction you need, and it costs far less clear coat.
Rupes DA Fine and 3D ACA 520 are finishing polishes. Rupes UNO Pure is the one to reach for when the paint is nearly perfect and you want the last five percent.
When a one-step makes sense
A one-step, or all-in-one, is a medium abrasive that cuts moderately and finishes well enough to stop there. On a daily driver with light to medium swirls, it saves a full pass. 3D One is the hybrid that adjusts its cut with pad and pressure. 3D Speed adds a wax so the car leaves protected.
How to decide
Wash and decon first, then look at the paint under a strong light. Light swirls visible only at an angle: polish. Swirls you can see from across the garage: compound then polish, or a one-step if you need to be done today. Scratches you can catch with a fingernail are through the clear, and no bottle fixes those.
Test a small section before you commit to the whole car. Every paint is different, and the test spot tells you more than any label.
Everything here lives in Compounds & Polishes.