How can I determine what would cause unexplained network traffic?

In the admin of my Unifi router I see constant 15-30 Mbps upload traffic. When I look at the connected clients, none of them has much traffic. At the moment I have two Debian servers connected to the network, one laptop, two phones, and several IoT things like a Eufy camera (not doing much), Apple TV (off), stereo (off). The Unifi router is connected to the modem. The tv box is connected to the modem directly, and tv is off. How can I find out what this traffic is, which device?

Question by SPRBRN on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.