Shielded rewards are distributed to all users in the shielded set. Users are rewared NAM tokens for each epoch their incentivised assets are in the shielded set. Informally, the amount of NAM rewarded for a specific asset is proportional to the amount of that asset in the shielded pool. Each asset has a "reward rate" specified by governance, which sets the constant of proportionality for that asset.
The interested reader should consult the specs for a more precise explanation of how the reward is calculated.