Septic tank for toilets.
Sinks, showers, washing machines to soakaway or grey water drain.
No detergents or bleaches should ever enter the toilet.
From your photo it all looks connected correctly, so the problem is either a blocked sink pipe, or a soakaway that doesn't soak away.
Just had to dig my sink pipe out, so many 90deg bends (7) the steel snake couldn't get own it. Now it only has 2 bends.