In your browser, what you have is an HTML input element. It’s the value of that element that you would set. The POST for that page would then send the contents of those fields to the server.
As confirmed above, yes.
No. You would not replicate the Player data in the Roster model. You would use the foreign key in the Roster model to refer to an individual Player.
Last Name is selected by the user, a coach creating a roster. First Name DOB, Last Team, Level and Zone are returned and injected into the form with a view and;
If you have an input element that looks like this: <input type="text" id="some_id" name="some_name"></input>
Then the attribute value is the value of the input field - in other words, something like: document.getElementById('some_id').value = whatever_value_you_want_in_that_input_field