Have a plumber snake out your drains.
ANS 2 - Install a flapper valve section on the sink drain pipe so water can go down but not up - cheap and very easy to do.
Blocked up main soil line (sewer)
because the upstairs toilet drain was clogged up.
It is quite possible that your main pipe to the city sewer system has become blocked somewhere between the basement floor drain and the man city sewer line. Could be a result of items flushed down the toilet(s), or damaged pipes as a result of age, tree roots searching for water, etc. Suggest having your main access to the city sewer system checked by camera.
partially clogged drain or vent, improperly installed vent or drain
There is a blockage in the waste line. Have you flushed the basement toilet to see what happens? Try running the upstairs shower and see what happens. (use two persons , one up stairs, one downstairs so you can control the amount of water that rises in the toilet.) There should be a "clean out" cap somewhere downstairs near the point where the waste line exits the house. It is here that one would "snake" the line. The other possibility is the water level in the downstairs toilet is lower than the level of the waste line exit. This means that the point of exit of the line is physically higher than the toilet. The only remedy for this is to raise the toilet to above the height or replumb the toilet into a waste pump that evacuates upwards to a point slightly higher than the waste line. This is the most common set-up in a "basement" toilet. The toilet evacuates into a point lower than the floor. The waste pump or grinder pump then pumps the waste upwards than gravity allows it to evacuate normally. The benefit is that the waste pump creates a separate system for the basement toilet. Y-THINK-Y
There is a main drain that runs from the basement through the roof where it is the vent for the system. If you look at the layout of the house, the upstairs bath is most likely over the downstairs one or the kitchen. If you look on the roof, you will see a 4 inch pipe coming through the roof that should be over the upstairs bath. The sink and shower drain into that and then to the basement and out of the house. Most likely the main stack is in the wall behind the toilet.
maybe bcuz to save water..
You have a grinder pump that boosts the sewage up to the main floor to enter the drain system. The float in this pump system is not working correctly and the standing water level in the tank is too high. Adjust the float, it will cure your issue.
I do not know if popcorn can be flushed down the toilet. So, I wouldn't try it.- Ashton Smith
Yes you can flush it. It's possible for a toilet trap to go dry after that much time. When you flush just make sure you check to see if the bowl is leaking.
The sewer
Depends on if it needed to be flushed or not.
Bad venting