I came to KiCad from Altium recently and I’m still learning. I am very impressed with version 6 and have donated to KiCad in the last drive to help out.
A few days ago, I struggled to draw a filled rectangle in a copper layer and assign a net to it, just as you described.
What I found:
You can draw a graphics shape in a copper layer.
Right click on it and select the command: [Create from Selection].
Now, you can make the shape into a zone, keep out, traces, etc.
Keep in mind that the original shape is still there, under the newly created one (You may want to delete it).
I tried it in PCBNEW and Footprint Editor and it works beautifully!
I hope it works for you too.