And:

I was searching for any native(non-proxied) YouTube client for Android in kotlin.

Fairphone is an option. Can be relocked. It is not important, what Fairphone supports but if it is supported by custom ROM developers. Fairphone is supported by most. But please avoid using Calyx or iode. They are both outclassed by Divest.

At least for /e/OS I can assure that relocking is simple enough. As there is partnership between Fairphone and Murena (/e/ developers) OTA updates are safe for future. Unfortunately the Fairphone support is horrible and /e/ is always too far behind security patch level.

Graphene only supports Pixel

Does not have swipe except when using a Google library

VPN is both, an interface on your device and a network connection (to a provider). Netguard only uses the first. So every request is going through Netguard but is not sent to a VPN provider, it leaves your device like it would if Netguard wasn’t active.

So no VPN provider can see your traffic.

While being sent through your local Netguard, the request is checked whether it should be blocked or not. This is done by its name. If the name is on a block list (real list or manually blocked) Netguard ensures that the request will end in nowhere otherwise it will go through.

It is not. Please educate yourself a bit more. They obviously cannot randomize all data. It is more a besteht effort approach.