SetActive or SceneManger?

For optimal resources when starting your game I would recommend to have a splash screen Which is a scene with maybe and image and a coroutine timer before loading the start screen.

You can continue this spash/load scene to have a seamless transition from scene to scene. Using SceneManager.LoadScene is far better than keeping scenes loaded in the background by setting them active false, as they continue to use up resources until unloaded.

Depends on the type of game your making sometimes we need to keep scenes loaded in the backgrounds to use global chats etc but once your more advance and probably publish a few games before then we might start to look into that.

Some games keep a menu scene active false thats the only time I would doit not with a heavy resource scene with many objects loaded.