There are two different types of bundles in my mind:
Charity bundles
Non-charity bundles
With the charity bundles, you usually don't get a money portion, or the money portion you do get is smaller. They're more for charity.
With the non-charity bundles, usually they take the money made from the bundle, and pretty much try to evenly (as much as possible) split it between all creators.
Bundles are also a process that involves at least two steps. The first step is submitting your projects to the bundle (simple process), and hoping they get accepted into the bundle by the bundle organizer(s).
The second step is that afterwards, at some point, you and other creators actually have to "approve" the bundle itself. This approval process is very important, so participating in it is important - or your submission(s) to the bundle might have to be removed so that the bundle can be launched. (Since launching the bundle requires that everyone participating approve it).
So with the charity bundles, it can be fun because you're helping out a cause, and getting more exposure to your projects.
With the non-charity bundles, usually you get exposure, and also get a portion of the money made.
Anyways, this explanation is just how I understand it. But maybe I should add that whenever I've submitted something to a bundle, the process of submitting it is pretty easy - so I think most people can probably figure it out.
This article might be helpful, too: https://itch.io/docs/creators/bundles.amp