How do I see the custom sky in texture packs?

How can I see the custom sky in Minecraft Bedwar Texture Packs?

I have already watched a lot of youtube videos but none of them show exactly why or how it works.

I always only see in videos how people have such a nice sky, but in Minecraft I always only see a blue sky no matter which texture pack I take.

I'm 100% sure that I have a texture pack in it that has this sky, because other Youtubers use this texture pack and have the custom sky.

I know that with the Badlion Client you can activate the custom sky in the settings. Maybe also with the normal MC under video settings