Is the ability to place bombs at the end of the spell Hexplosive Minefield a delayed trigger? When does someone choose which battlefield to place the bombs?

Yes — "at end of turn, play that many Bomb tokens to a single battlefield" is a Delayed Triggered Ability created when Hexplosive Minefield resolves, and you choose the battlefield at the end of turn when that trigger is finalized on the chain, not while casting the spell.

The two choices land at different times. How many Bombs you kill (and which ones) is a targeting choice made at finalization of the spell itself, so "that many" is locked in the moment Minefield goes on the chain. The destination battlefield is a choice belonging to the delayed trigger generated by the resolving spell, so it waits until the end-of-turn trigger is placed on the chain (Rule 355.5.b — choices for triggers generated by a chain item aren't made when that item is finalized).

Practically: your opponent sees which battlefield you picked before they respond, since the trigger is finalized (battlefield chosen) and then becomes a chain item others can answer with Reactions. And because Bombs have Deploy, "a single battlefield" can be any one battlefield — including an enemy-occupied one (Rule 830.1.b.1). One edge case: if the spell is somehow played after the end-of-turn window has already begun, the delayed trigger is never generated (Rule 359.3.e.16) and the Bombs stay dead.