So yes, I have several Ethernet ports next to power sockets in several rooms. The cables from these all go back to 1 room where I have the router and switch. Its a 16 port switch. Each Deco unit therefore gets the full BB capacity to turn into the mesh WiFi. Previous to rewiring the house I used...
It’s a choice, the other units can boost the same WiFi picking up the WiFi of the main unit, but they do work a lot better/faster if you can hardwire each unit. I had the house rewired anyway, so at the time put some cat6 in.
I think the above answer can work, but can also result in WiFi signals interfering with each other.
I have a Tp-Link Deco mesh units. The router is put into modem mode and has 1 Ethernet cable to the master deco. I then use a tp-link switch off the master deco for all hardwired connections...