The two parts of Alpha Brawl’s effect happen sequentially: first the targeted creature deals damage, then the other creatures deal damage. State-based actions aren’t checked in the middle, so creatures that are dealt lethal damage by the targeted creature will still deal damage.

None of the damage dealt due to Alpha Brawl is combat damage.

Combat abilities like first strike, double strike, and trample don’t apply and will have no impact on Alpha Brawl’s effect.

Abilities like lifelink and deathtouch that care about any kind of damage are taken into account.

If the targeted creature has infect or wither, the damage it deals will result in -1/-1 counters being put on each creature it deals damage to. The power of those creatures will be reduced when they deal damage back to the targeted creature.

The damage dealt by the targeted creature is not divided in any way. For example, if a 3/3 creature is targeted, it will deal 3 damage to each other creature controlled by its controller.