Does Alpha Strike's damage assignment happen before or after finalization?

Alpha Strike’s damage assignment happens after finalization.

According to the Showdown Flow and the Core Rules regarding the FEPR process:

1. Finalization (Rule 337/355.14): When you play Alpha Strike, you must first finalize it on the chain. During this step, you must choose the friendly unit that will deal damage and the enemy units that will receive the split damage. These targets are locked in once you finish finalizing the spell.
2. Resolution (Rule 157): Damage assignment only occurs when Alpha Strike actually resolves (i.e., executes its rules text).

Per RiftJudge FAQ #9355, you choose your targets during the finalization step. You cannot change those targets once finalized, and the damage distribution is calculated at the moment the spell resolves based on the game state at that time.