The newer edit wins, judged by each item's own edit timestamp. Devices converge on the same result whatever order they happen to sync in, and being days late changes nothing.
Three cases cover everything.
Different things changed on each device. Both survive. Add a card on your iPhone while renaming a subject on your iPad, and you keep the new card and the new name. This is by far the most common case, and nothing is lost.
The same card changed on both. The newer edit silently replaces the older one. There is no conflict copy and no prompt. This is the one place where a change can be lost, and it requires editing the same card on two devices without syncing in between, which is rare in practice.
Edited on one device, deleted on the other. The newer action wins. If the two happened at the same moment, the deletion wins.
Open Reps on a device while it is online before switching to another one. A few seconds of sync is enough, and it removes the only scenario where work disappears.
This matters most if you keep a device in airplane mode for long stretches, or if you write cards on a Mac that stays closed for days at a time.
Answering cards on two devices does not conflict in any meaningful way. Each answer updates that card's schedule, and the later answer is the one that counts, which is the correct outcome.
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