Example :

Ground floor - Main Router

1st floor - mesh router (connected wirelessly to Main Router), can this mesh router connect to a PC with Lan cable?
or
1st floor - mesh router (connected via Lan cable to Main Router) better?

I recalled reading somewhere, that mesh router can be connected to Main Router via both wireless or Lan cable also can right?
& mesh router can be connected to PC/devices via wifi or Lan cable also can, correct?

Yes, means, Mesh router connected to Main Router via wireless, then PC connect to Mesh Router via cable.

So from the explanations above, apparently, all routers can be connected via wireless or wired also can, and all devices can be connected to Main Router or Mesh Routers via wifi or wired also can.

Excellent, means Mesh is perfect. 👍