A YouTube video would be hosted in something like an EMBED or OBJECT tag. WebDriver can locate these tags inside the DOM. It would be able to tell you the size and position of the object but not much beyond that. If you have a tool/library which can manipulate the YouTube video, you could pass the size and position using WebDriver but a third party tool/library would be needed.
Not sure what you are looking for when you talk about dynamically changing text. If you mean you want a different text string on every run of an automation suite then you should look into the system time. In Java I would use the System.currentTimeMillis() to get the current time. Usually at the start of a test I would get the current time, print it to a log then use it for various strings, e.g.
String timestamp = new String(System.currentTimeMillis());
String username = "Username-" + timestamp;
Darrell