Omniverse is what you get when you sort multiple multiverses, that's all. It's a way of categorizing multiverses sharing some common element. Because infinity is so vast that it's basically meaningless to us humans, so organizing it at least makes it vaguely easier to understand.

The dumbest and easiest way to understand it is with media franchises as an and analogy. All of Marvel is one multiverse, all of DC is one multiverse, all of Terminator are another, all of Star Trek another, etc. it's sloppy but here's my point across.

In reality it's more like; this multiverse has universes with identical physical parameters, that multiverse has a slightly higher amount of gravity, that multiverse has a slightly different amount of weak magnetic force, that multiverse has a different speed of light, etc.