Because I tried playing a YouTube video and didn't work.
Given that you have the composite cable and can't output the picture of the YouTube app, this must mean you try to use the AppStore one.
Solution:
1, if you're still on iOS 5.x, use the stock player instead of the AppStore one. It supports native TV out over the composite / component cables. If you insist on sticking with the AppStore version, see Bullet 2 instead.
2, if you're on iOS 6+ where there's no stock YouTube player any more meaning you
must rely on the AppStore version, purchase
Resupported 4.0+ from Cydia Store. As of the current version (1.8), it adds TV output to (among other ones) to the AppStore YT app.
NOTE: should you want to connect a HDMI or a VGA cable when Resupported 4.0+ is installed, they won't work. (The RGB cable will.) In these cases, you will need to uninstall Resupported 4.0+. You can safely reinstall it after finishing using those high-res adapters to be able to output YT videos over the composite / RGB (component) cables again.
NOTE 2: I don't know whether there are any AppStore / Cydia YouTube players with native TV output capabilities.
NOTE 3: I've just tested this on my 6.1.2 iPad 2 with my (stock, genuine) RGB cable and the Appstore YT app.
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NOTE: should you want to connect a HDMI or a VGA cable when Resupported 4.0+ is installed, they won't work. (The RGB cable will.) In these cases, you will need to uninstall Resupported 4.0+. You can safely reinstall it after finishing using those high-res adapters to be able to output YT videos over the composite / RGB (component) cables again
Regarding the above problem, I've just sent the following message to the dev. Hope he answers / does what I've recommended:
Dear Paul,
I'm working on an all-in-one article on everything TV output for my mostly multimedia-related iPhone Life blog.
I'll also devote a lot of discussion to your tweak. (And will definitely recommend it - as I've done at, say,
https://forums.macrumors.com/posts/17193248/ )
There is a HUGE problem with the tweak, however. If you connect a HDMI or a VGA cable when Resupported 4.0+ is installed, they won't work, only the composite / RGB cable will. That is, you will need to uninstall Resupported 4.0+.
This happens on all iDevices, regardless of their model or iOS version they run on.
This is pretty awkward. Isn't there any way of temporarily disabling your tweak so that the high-res adapters can also be used?
Then what do I do next? Do I need some Cydia tweak or anything? Because I tried playing a YouTube video and didn't work.