To make/ develop any app, first you will have to take app development course. There are a plenty of websites which provide such courses. So, you are in class 12th, first of all, focus on your studies and if you are done with it, you might take such course. After taking the course, you will have every information of about how to make an application. However, if you also wish to make a website (like OYO has), then for that you will have to complete web development course. However,  you can also use HTML, thereafter using a host provider and a domain name, you can publish your own website on the internet.

Hope it helps.

hey,

i hope you are doing great see the thing is that there is no such code by using which you can develop an application like OYO , the foremost thing is that to develop an application you need to have very well command on the coding and should have excellent knowledge of coding , its not that easy as your thinking to develop an application.

being a student of 12th standard you can only develop an application if you have sufficient knowledge about coding

hope this helps

good luck!