Is semen rendered impure by the presence of a drop of urine at the tip of the penis upon its emission? And is semen pure or impure after falling on the bathroom floor and being walked upon, as well as when it falls on a mobile phone or the body?

If you are afflicted with waswasah (obsessive whispering), then disregard it. If you are certain that the glans of the penis has been contaminated with urine, then the semen that comes into contact with it becomes impure. The jurists have stated that the semen of one who performs istijmar (cleansing with stones) becomes impure due to its contact with the urine on the glans of the penis; thus, it is even more so for one who has not performed istinaj (cleansing with water) or istijmar. Al-Bujayrami said: "And the semen of one who performs istinaj with stones becomes impure, and for this reason, it is forbidden for the aforementioned person to have intercourse with his wife, because the forgiveness for it is only in relation to himself." So, if the semen becomes impure, it contaminates whatever part of the body or clothing it touches. As for what you doubt has come into contact with it, the default is its purity, so its impurity is not ruled by mere doubt.