What is "custom range scan"?
It is a page that allows you to find a specific address between the starting and ending HEX values you enter. You can also enter the maximum number of keys to be created for each creation. By default it searches within the beginning and end range of Bitcoin. The goal here is not to find a private key. It's a little research and fun.
If the wallet address is found, it will show you all its information on the screen. It is not recommended to use this page to search for puzzles. Although the probability of being found through the browser is directly proportional to your chances, it is very, very low.
Remember, there are almost infinite possibilities of (theoretically) private keys, and while finding a private key is theoretically possible, it is impossible in real life. You can use this page offline.