I know installing an app thro other places like pc/web is possible , Sideloading android From Your PC or possible through manual login into google play and installing.
Is there a way to achieve the same through code/google play APIs? I dont want to use adb or unknown sources.
E.g I want to install/push watsapp thro code from my pc/web
Yes that's possible, but not officially.
You can login to the google play website from code and simulate a button click there. This is a sample application doing exactly that, including login via device's google account. I don't know if it's still working, cause it's build against the old google play website, but I assume the source code could be helpful anyways (documentation is in japanese, use google translate for that).