All apps on smart TVs are browser apps. Some, like Youtube, get special privileges (that is, can run their own app), but also need to run some kind of UI since TVs have no standard UI libraries. Youtube decided to run a stripped down version of Chrome (Cobalt). Others (IIRC Netflix) run their own renderer. Or do weird stuff, see Comcast and Lightning https://www.lightningjs.io/
TVs own interfaces are some bespoke one-off stuff that isn't exposed to other apps.
And this sucks because Raspberry PIs have more powerful hardware than even modern high-end TVs.
There's some exceptions to this rule. Google TV devices are Android under the hood and thus, the TV apps are Android apps distributed as apk's just like on smartphones (not web apps).
Most of those are webapps too (just packaged in APK) since it doesn't make sense for most app developers to build a special bespoke version for Android.
If you check YouTube version string you'll see Cobalt as well on Android.
TVs own interfaces are some bespoke one-off stuff that isn't exposed to other apps.
And this sucks because Raspberry PIs have more powerful hardware than even modern high-end TVs.