How does Asynchronous Javascript Execution happen? and when not to use return statement?

// synchronous Javascript
var result = db.get('select * from table1');
console.log('I am syncronous');

// asynchronous Javascript 
db.get('select * from table1', function(result){
    // do something with the result
});
console.log('I am asynchronous')

I know in synchronous code, console.log() executes after result is fetched from db, whereas in asynchronous code console.log() executes before the db.get() fetches the result.

Now my question is, how does the execution happen behind the scenes for asynchronous code and why is it non-blocking?

I have searched the Ecmascript 5 standard to understand how asynchronous code works but could not find the word asynchronous in the entire standard.

And from nodebeginner.org I also found out that we should not use a return statement as it blocks the event loop. But nodejs api and third party modules contain return statements everywhere. So when should a return statement be used and when shouldn't it?

Can somebody throw some light on this?