Q: Why did he rebelled against the angels?
A: Because MC found out later that both angels and demons are equally contemptible beings unworthy of any trust—well, except for one angel.
Q: Are they actual angels like the one from the Abrahamic pantheon or just an alien civilization?
A: Author clearly borrowed many names from the Bible—for angels and demons. But they are more like extraterrestrials with familiar aesthetics and names. I'm a bit hesitant to call them aliens or extraterrestrials because they look humane and act humane. It's almost like they are some kind of convenient plot devices and nothing more. And questions like this pierces through the main problem of this novel; Other than MC, everything's an afterthought.
Q: Do they even follow God or is just they own desires?
A: Supposedly, angels and demons have been fighting forever. At one point, demons overwhelmed angels and angels retreated to their safe haven; Heaven. Demons can't enter angels' realm without a key, Zero Code. Obviously, angels want the key as well but they can't roam around freely looking for the key because of demons. So angels decide to summon humans and grant them the game-like system, to make them fight demons in their stead. If there was any god-like figure angels or demons served, the opposition would scale up rather than scale down. Angels would have no reason to rely on flimsy humans anymore and there won't be OP MC we, as readers can spectate. So, long answer short; they don't follow no god and they are on their own. Just think of it this way. There is no male angel in this novel. There is no explanation provided. But boi these angel tiddies look good or what. It's a webnovel. Author doesn't bother to explain and we don't bother to understand.
Q: Also does God exist in the Memorize universe?
A: Yes. There are many gods and our OP MC started New Game+ with a certain goddess' power. He is quite not yet possessing Divinity but is and will be borrowing her power until then.