What is the order of cleaning up a combat step, including when units go to trash, when Deathknell triggers, and when Zhonya takes effect?

Ruling: Units go to trash during step 2a/2b of cleanup. Deathknell triggers and becomes pending when cleaning up units that have taken lethal damage, then finalizes and goes on a chain before step 2 of combat resolution. Zhonya triggers as a replacement effect when a unit dies during cleanup.

Sequence:

• Step 1: Combat resolution occurs
• Step 2a/2b of cleanup: Units go to trash
• During cleanup: Deathknell triggers and becomes pending for units that took lethal damage
• Before step 2 of combat resolution: Deathknell finalizes and goes on chain

Nuances:

• Zhonya is a replacement effect that ignores the chain, so it triggers immediately when a unit would die during cleanup
• If Zhonya saves a unit, Deathknell will not trigger for that unit since it didn't actually die