What happens when you connect two smartphones that have reverse charging?

When you have two smartphones that both have reverse charging, what happens when you connect them via their USB ports? Which one charges which? Does the one with the bigger battery do the charging? My primary phone, a Xiaomi Mi Note 10 Pro, was running out of battery juice, and I had a Nokia 5.4 with almost full battery lying around. So, I plugged them to one another and was pleasantly surprised. The Nokia 5.4 began to charge up my Note 10 Pro. It looks like, by default, the phone with more juice charges the one with less. is there another explanation?