Probably the most popular related project with GUI is Wireshark
Wireshark is the world’s foremost and widely-used network protocol
analyzer. It lets you see what’s happening on your network at a
microscopic level and is the de facto (and often de jure) standard
across many commercial and non-profit enterprises, government
agencies, and educational ...
You have to configure the RasPi as a simple router. With tethering you should always get an ethernet interface. For example with USB-Tethering you get the interface usb0 out of the box with Raspbian. You can use this interface like any other network interface. I use systemd-networkd to configure networking because it makes things easier.
Just follow to Use ...