How are applications made that can perform some kind of manipulation on sites?

Question

Mobile development

There are applications that allow you to do various kinds of manipulations on the site. Installed, went to the same chrome and do what you need. I will give a primitive example that I would like to implement now: You
install my application -> for example, by swiping, change the color of the block of any site that you are viewing, no matter what browser.
Where to look to implement this? Is it possible to write this on xamarin or phonegap?

Answer 1

It's easy if one of the 4 things is:
1) If the site itself supports it in its API
2) If this application is not a third-party one, and there is no plugin / extension installed in this particular site
3) If this application is a proxy server
4) If this app intercepts system webview hooks