Solved
How can I get deep study into objects/methods in Vuforia Studio JS environment?
Such like tml3dRenderer.setTexture, document.getElementsByClassName, etc.
Is there a guidance for new users to approach to them?
There is a decent amount of AngularJS & JavaScript documentation available on the web.
For JavaScript you can check out w3Schools: https://www.w3schools.com/js/js_htmldom_elements.asp
For AngularJS: https://docs.angularjs.org/api
Try printing the $scope object to your browser console to explore.
console.log($scope);
This will help you identify available functions, properties, etc.