star trek - Do we ever learn what the Prophets did to the Dominion Fleet?

We don't know, nor is it ever stated within the main canon (TV, Films) what happened to the Jem'Hadar fleet.

The script notes state:

The approaching Dominion reinforcement fleet has begun to be hit by
some sort of strange energy jolts that crackles from ship to ship. And
as we watch, the ships seem to vanish until the entire fleet has
disappeared.

...

GARAK: Then where did they go?

Sisko begins to understand what the wormhole aliens have done.

SISKO: Wherever they went, I don't think they're coming back.

And that's literally your lot.

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Moving down the canon scale, the fleet reappears in the future in the Star Trek Online mission "The 2800", later retitled as the "Cardassian Struggle".

In this mission, The Dominion fleet suddenly materialises around 40 years into the future and proceeds to board DS9 and take the crew hostage. It's up to you, the player to liberate that crew with the minimal loss of life.