With LGTV you could launch an application by setting the AppLauncher item. Just set it to “netflix” or “amazon”.
To command it via google assistant, you’ll need to create one virtual item called “Netflix” and another virtual item called “Prime”. Then you can tell google assistant to “Turn on Netflix”, and you write a rule that turns the TV on, and launch netflix. If you use metadata on the item, and group them, you only need to write one rule for both netflix and prime and any other app you may want.