Web Crolling & API questions. Change data in my app & Send to original page & server issue

There are hotels and users can reserve through the app.
But the problem is that not every hotels have it’s API, so I’m thinking of crawling.

I am a beginner at this, and here’s the question.

  1. API - how to understand GET/POST? - how to use?

  2. crawling - can we change the data of it in our app?
    ex. if user reserves, can it display as ‘reserved’ in our app?

  • I don’t think changed data can be sent to original site, but any way to do this?

Another question …

If crawling have an issue with server over load of the original page, Why API don’t have problem with it?

These are my overall curiosity, and I would appreciate so much if any one can answer these.