What use for the thin, folded, two-side protector that was boxed with my iPhone XR? The iPhone XR that I recently bought from Apple came with a thin folded plastic protector that covers both sides. (In what follows I call this the Apple protector.)

I have also bought (again from Apple) a separate and (presumably) tougher screen protector, namely the Anti-Glare Screen Protection badged by Belkin; and I have bought a protective case made by Speck, the Presidio Grip, which is described as compatible with screen protectors.

Before I fit the case, should I fit the Belkin protector on top of the Apple protector, or the other way round? Or should I take the Apple protector off and keep it in the event that the Belkin protector gets damaged or mislaid in the future? Or perhaps the Apple protector no longer has a purpose and I should dispose of it?
Your "Apple Protector" is just a cheap piece of plastic made for disposal:
it's just there so you can remove it and view your new phone in its glory without fingerprints.
Apply the real (Belkin) protector immediately for protection.

Proof: the Apple Protector usually doesn't have openings for the earspeaker part.