When flushing toilet upstairs toilet in basement overflows from bottom onto basement floor?

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