According to the MWI, does the universe always split with all possible outcomes or only splits for those particles that were observed? Sorry in advance if this is a stupid question.

The Many World Interpretation (MWI) says that, at every point in time, the universe splits into a multitude of existences in which every possible outcome actually happens in parallel.

According to the MWI, does the universe really split at every point in time with all possible outcomes, or does it only split in relation to the objects that are observed?
As I understand (and I don't understand much TBH), according to the Copenhagen Interpretation, only one of the possible states "is picked" when you observe a particle (the waves collapse and so on). And so far, I thought that, according to the MWI, universe splits would only happen when particles were observed, similarly to the idea from the Copenhagen Interpretation. But by reading the quote above, I'm not sure anymore.
As an example: is the cat always dead and alive in different branches of the universe regardless of being observed or not, or does the universe only slip when someone looks at the cat?
 A: The key to understanding the many-worlds interpretation is understanding that a measuring device ends up in a superposition state to an outside observer that is closed off from the device. See the Wigner's friend problem for a more detailed explanation of this.
To an observer outside the universe who has been isolated from it since the beginning of time, only unitary evolution has occurred, and therefore the universe is in a superposition of all possible outcomes. The many worlds interpretation is that each of these possible outcomes are real, and that if this observer were to check the universe -- he simply end up checking which of these branches he is in.
The Copenhagen interpretation, on the other hand, would say that all of those superposition states didn't really exist and didn't have meaning, and that the universe just becomes the possibility that that personal collapsed it to. This is a weakness of that interpretation, because you can only explain what happened in the perspective of the people who are in the superposition only retroactively after that outside observer has collapsed the state.

As an example: is the cat always dead and alive in different branches
of the universe regardless of being observed or not, or does the
universe only slip when someone looks at the cat?

Always alive and dead in both branches. The act of checking the outcome of the cat is simply figuring out which of the universes that observer is in.
