My parents are from Hong Kong and they never learned Pinyin so they can't type Chinese with the keyboard offered by the app.
I managed to connect the phone to TV and use the remote control function to use the hand writing function from the phone but it doesn't work with YouTube (and Netflix apparently). Is there any way to choose keyboard in those apps or another way for my parents to handwrite?
The most polished way is to use Chromecast from the Youtube app on a smart phone. This way they can enter text the way they're used to on their phone.
I believe Google keyboard (Gboard) is also installed by default. You can install languages via Settings -> Device preferences -> Keyboard -> Gboard settings -> Languages.