I wanted to ask that is the water used to wash a najis thing pure or impure?. For example, if my lower thigh becomes najis and I use water to purify it, and the water splashes and slides down my leg, now if the water has became impure, my ankels, and foot must have became najis now? and also the tiny drops of water which splashed back when I washed my thigh when landed on the ground made the ground najis right? Thus my whole bathroom becomes impure when I try to make any part of my body pure?

If you are using water from a hose, then it’s pure and no need to worry about anything becoming impure. But if you are using water from a water can, then yes the water which comes into contact with the najes area becomes najes and you must try to avoid having those drops fall onto another area of your body. As for the water splashing, if you don’t have certainty that those drops which splashed are najes, you may consider them pure.