There are two different types of eeros: eeros and eero Beacons.
eero (2nd generation)
eero (2nd-generation) has the same elegant design and footprint as eero (1st-generation), but is more than twice as powerful. It is the first and only WiFi access point of its size that can broadcast on three wireless bands simultaneously (tri-band WiFi), doubling the speed of its predecessor. As the gateway of an eero WiFi system, eero has two auto-detecting Ethernet ports. One connects directly to the modem and the other port provides the option to hardwire devices like an Ethernet switch, printer, or video game console.
Further reading: What's the difference between the 1st and 2nd generation eero?
eero Beacon is half the size, but even more powerful than eero (1st generation). After a year and a half of collecting customer insights and feedback, eero found that more than half of customers place eeros in stairwells, kitchens, and hallways, where cords are disruptive. With eero Beacon, simply plug each unit into a wall outlet to easily expand coverage in the home. Add as many eero Beacons as needed to provide each room with WiFi coverage.
Are eeros interchangeable?
In every network, you must use an eero as your gateway (the eero connected directed to your modem). This is because eeros have Ethernet ports on the back, which are required for creating a connection to your network. You can swap out your gateway eero with a 1st or 2nd generation eero (just not eero Beacon).
As for the rest of your network, you can use any eero (1st, 2nd, and eero Beacon) as your additional eeros. Should you wish to hardwire your eeros, you will need to use an eero, as well as for networks where you want to connect certain devices via Ethernet.