Understanding a Powerful Story of Forgiveness in Islam

Assalamu alaikum, I wanted to talk about that story of the person who had done so much harm and then was granted forgiveness. Honestly, it leaves me a bit confused. Like, how can we be certain that Allah accepted their repentance? They did repent and turn back to Allah, but still, where's the absolute assurance? Also, I keep thinking: if we wrong someone, don't they have the right to justice on the Day of Judgment? Here, so many were wronged, so how exactly does forgiveness for the wrongdoer work? Won't those who were harmed get their justice? I remember hearing that if someone doesn't forgive you, it might affect things, so I'm just trying to piece it all together. If anyone can help shed some light, I'd really appreciate it. The references are from well-known authentic collections, but I'm struggling to reconcile this with what I've understood about justice and forgiveness.