How to download apps above 100MB through cellular data instead of wi-fi..??

Some apps will download if they are over 100mb but you will know that when you try to download it and see a message (I'll post a screenshot of what the message says. If you click ok you will immediately be able to tell if the app will be allowed to download by seeing the blue start to show. Otherwise you can't.

You can't.

You can't, if you have a 2nd phone or any other device, create a personal hot spot or tethering.