Which way is the best of making levels in endless side scrolling games?

Hi guys!

I'm a beginner at programming but I have some basic knowledge and I'm making simply prototypes of simply games. I have an idea for simply game, like endless runner / flyer (flappy bird) but with original mechanic (I think so). And now, I'm wondering, how to get the best way to make / design levels for that game. It will be obstacles, maybe enemies and other stuff like that. Now I have the main player object with movement script component , so the test level is made as an big (maybe very long is better word) object. The camera is following the player i X axis. But I'm just thinking that the player should'nt move, rather that the background with obstacles should be generated on the right side (out of the visible game screen area). But how to "manage" that in code scripts ? It should be like: Obstacle 1 set spawn position on the right side after 5 seconds of starting game, then set Obstacle 2 spawn position after 7 seconds ? Or maybe in more "clear" way.. ?

 

https://unity3d.com/learn/tutorials/topics/2d-game-creation/project-goals?playlist=17093