earthship biological tank for waste water/ stupid questions

Dear Permies,

I would like to know if anyone has built an Earthship sewage system. Im in Africa and wanting to do one. I will have to buy a plastic tank and put a drainage valve on top. My specific questions are these:

1 After installing underground and connecting the drainage pipes from the sink, is the tank filled with earth? On all the videos it shows plants growing on top of the tank. or do they grow in the water alone?

2. How is the tank covered if plants are growing in it?

3. Is there drainage on the bottom or the grey water stays in the tank until the plants absorb it?

4. will any large plastic tank with overflow drainage valve installed on top do?

im in Kenya and left my earthships books volume 1, 2 and 3 at home by mistake.

Sincerely,
Dori