Just place planes.
If the GND plane will have the lowest priority than it will be filled after the others so will fill the rest (unused by other ones) of PCB area.
They are called “copper zones” in KiCad and it is in: Pcbnew / Place / Zone There is also an icon on the right toolbar for it, just next to the layers manager (Watch their tooltips).
When placing a Zone there is a spot to say this zone is “ground” or what ever node you pick. If the net name is not right it will not connect to the right pins of the IC.
There are 3 different ways to view the zones. That will need to be set.
You should watch a youtube video about this and watch some one do it. link, click here