Thank you @ScoffingLizard@lemmy.dbzer0.com for telling me about the pcap app! On rooted devices, you can even be running a VPN simultaneously, because then the pcap app doesn't need to take that slot.

Please share any knowledge or experience that you have on packet inspection using smartphones. I would like to sniff at my switch, which is switching between my (two LANs)^[I put "two LANs" within parentheses to denote that the two LANs are not being switched between.] and my ISP. For this, I have mirrored all ports that are in use to a port that is currently not in use. Can I simply buy one of those USB-C to Ethernet dongles and use tcpdump in Termux? Will Android recognize the dongle as a sniffable network interface? The phone is rooted.