This setting is in the bulk placements editor:

  1. Open Meta Ads and click Bulk edits, above the table.
  2. Select the ad sets and, in the bar that appears at the bottom, open Edit Ad Set → Edit Placements.
  3. If you only want to change the device targeting, choose Don't change placements (each ad set keeps its current setting).
  4. In the Specific mobile devices and operating systems block, choose All mobile devices, iOS devices only, or Android devices only (Do not change keeps each ad set's current setting).
  5. Optional: under Included devices and Excluded devices, enter the model (the list comes from Meta's catalog), and under Operating system versions, set the minimum and maximum version.
  6. Save.

If you choose no model, the ad runs on all devices of the selected operating system. Before sending anything to Meta, Ads Editor checks that the combination makes sense (an iPhone model with Android, for example). The same setting is available through MCP.