FTL will determine the current IP bound to the interface the query arrived on. (This does not mean your device doesn't need a static IP!)
https://github.com/pi-hole/FTL/pull/1111
It does this only in cases dhcpcd5 is already installed (i.e on Rapsberry Pi OS). For all other distributions you need to set a static IP manually.
https://github.com/pi-hole/pi-hole/pull/4260
Pi-hole is not meant to be a network monitoring tool. But Pull requests adding this (back) are always welcome.