ip link show
gave me an overview about the network adapter names of my computer.
From observing results of
ip link show with the LAN pluged/unplugged and the WLAN enabled/disabled, I concluded that in my case
enp0s25 designates my LAN adapter and
wlp3s0 designates my WLAN adapter.
But the sampling time of 0.25s was a bit too short. Setting it to 1 second made the activity bar show up when I let firefox show a Bing map. One should set the upper scale ends for incoming and for outgoing traffic to what one expects. I tried it with 50 KiB/s incoming, 1 KiB/s outgoing because in most cases my question is: "is there any network activity".
It would be nice if the scales could be made logarithmic (however since the logarithm of 0 is -Infinty, clipping off all network activity below one packet per sampling)!
It is possible to place e.g. two instances of Network Monitor: one for WLAN, the other for LAN. I called them W and L. - Thank you, HattinGokbori87!