You may noticed that the kitchen sink and lavatory both have p-traps underneath, instead of just a straight one. The purpose of these p-traps is to block the smell. If these pipes are just straight ahead, the smell of the discharged liquid will continue to come out of the drain of the sink/lavatory. Hence by curving the tubes, it will block the smell from coming out of the drain.
Also you may have noticed that the p-trap in the lavatory has a little circular on its curved area which is called cleanout. The purpose of a cleanout is—from its name—to clean the tubes. A brush can fit through the area and through that, you can scrub the insides of the tube.
But in kitchen sinks and lavatories, cleanout has another purpose—to obtain an object that fell of the drain. This is another reason why p-traps are important. In case that something valuable fell of the drain (such as ring or earrings) it does not directly go to the septic tank or sewer line or cause problems inside the drainage system, it will end up in the curved part of the p-trap and you can obtain it through the cleanout.
What is commonly known is bidet is actually shutoff spray (though the shutoff spray has almost the same function as the real bidet). Then there is the faucet, floor drain, and water closet (aka toilet!).
I am currently learning more pumbling-related concepts (it is super interesting!!!) and if there is anything that you would like to add or correct from this post, please feel free to comment. I still have a lot more to learn and any new information would be great!
Hopefully this post will encourage you to notice things around your home too!