Does the cleanup step use the stack or do effects just happen automatically?

The cleanup step is unique because its two main actions happen automatically, without using the stack. First, if the active player has more cards in hand than their maximum hand size (usually seven), they discard down to that limit. Second, all damage marked on permanents is removed and all 'until end of turn' and 'this turn' effects end. These happen simultaneously and cannot be responded to (CR 514.1, 514.2).

Normally, no player receives priority during the cleanup step at all (CR 514.3). This means players cannot cast spells or activate abilities during a typical cleanup step — things just happen automatically as the rules dictate.

However, there is one important exception: if a state-based action would be performed, or if a triggered ability triggers during the cleanup step, then a new cleanup step begins after players receive priority and the triggered ability or state-based action resolves (CR 514.3). This can chain into multiple consecutive cleanup steps if triggers keep firing.

Example: You end your turn with 8 cards in hand. In the cleanup step, you automatically discard 1 card to reach 7 — your opponent cannot respond to your discard choice with a counterspell or instant, because no one has priority at that moment.

So the cleanup step is essentially automatic housekeeping. The stack is only involved if a triggered ability fires as a result of those automatic actions, forcing an extra cleanup step afterward.