What should one do if a drop of urine fell on the bathroom floor and he forgot to purify it, then remembered and purified it, but he does not know if someone stepped on it while it was wet and then left the bathroom? And does that contaminate the rest of the apartment floor?

If water is poured over a spot where a drop of urine fell, it becomes pure due to the abundance of water poured over it. Doubts about impurity transferring via shoes are not to be considered, because the default state of shoes is purity unless it is known for certain that they have been afflicted with impurity. One must be wary of continuing with obsessive doubts (waswasah).