I wanna take up this challenge. I use to put 30 pounds on my back and do like 15 push ups. I heard doing pushups with heavy weight on helps gain muscle more and doing it without the weight would increase your endurance.
Well, it depends on the intensity at which you do them. You can build muscle if you increase the intensity and reps (doing variations of push up will also help; wide, knuckle, clapping, elevated, one arm etc.).
As long as you get decent form down, with or without those modifications, you should be fine and YouTube can always assist with showing you what that is. GMB Fitness is a decent channel to teach you.
Still good. As you do it, get to 50 first and then 75 and maybe 100 all at once in a couple months. It's hardcore. I am more like on 50 now, but always used to do 75.