The EERO Pro, like the EERO Home Wifi System, both transmits and receives over WiFi -- no Ethernet or Powerline adapters necessary...essentially, your "main" unit (with Eero gen 1 or Pro, it can be ANY unit) connects as the "main" router to your modem...every Eero Gen 1 and Pro comes with two ports which can be used as incoming, or outgoing. With the beacon system, the main router is the non-beacon.
You can then add on Eero Gen 1s, Pros, or Beacons to your network...any (logical) number of Eeros can be set up in tandem and, furthermore, a box of three EEROs is NOT a married pair. I've installed anywhere from 1 to 9 in a single home, depending on your needs...if you get a 3-pack and only need 2, the third can be used at an entirely different location as a single unit or added onto a different set (I.E. - 1-EERO studio or 4-EERO office)
They all communicate over WIFI, however, in pre-run wired homes, each one can be plugged into the ethernet network. Albeit, this is unnecessary. Likewise, PCs, TVs and other devices can be plugged into an EERO (satellite or main) and will act as if it is on LAN. Think of satellite EEROs as range extenders / access points, HOWEVER, due to their smart appliance nature and chipset, there is essentially no handoff delay and your entire network is seamless. Furthermore, they adapt to their unique install environment to best utilize each device.
1-Make sure that you ping the install locations and each unit has a strong connection to one another.
2-Don't cheap out and install less than is necessary
3-Likewise, don't overdue it and add too many, as this will cause interference.
4-The Gen 1 or Pro system is, in my professional opinion, far better than the beacons in terms of versatility -- the beacons are designed almost exclusively for hallways / staircases.
5-You can add multiple powerline adapters onto an eero network if you really need to....only in cases of metallic flooring / walls or a concrete basement was this necessary.
*5+ years of in-home and business technical support, smart home, AV, security and network installations in the LA / OC California area.