>>My phone does not have modified version of Android
Most phones are shipped with a modified version of Android.
Basically Google (who make Android) produce a "pure" Android and then give it to Samsung, HTC, Sony etc who then all modify it, change some icons, add or remove some apps or features and so on, so they can ship their own "tailored" Android.
Google Phones (Pixel XL for example) run pure Android, and I think the new Nokia Android phones run pure Android, but most other phones don't.
This is why there is a delay in shipping the latest version of Android on phones like Samsung etc., it is because they are tailoring and testing the new version before they ship it.
It is also why older phones don't run the latest version of Android, because it is a lot of work for companies like Samsung to do it, and on older phones it is not worth it.