Originally Posted by
OldWestPaladin
I don't think I'd count the time travel ones. I'm pretty sure marvel differentiates between timelines and the various "earths". Also different are the ups and down "dimensions" like limbo, and the magic stuff.
Hickman laid out the architecture.
The parallel universes are like slices of bread in a loaf. This is surrounded and permeated by the superflow. The Ultimates have traveled to outside the superflow.
Time, and branching get timelines, move along the T-axis, not the parallel space dimensions. You can only travel to different dimensions if they are on a branch from your starting point. multiple futures, immutable past.
The other concepts are above and below (heaven, hell, limbo, crossroads, etc), and pockets (darkforce, white space, some of the old concepts from Quasar). And then there is the expansion of each universe, outward, encroaching on both the negative zone and the Oblivion entity's nothingness.
So the major current paths "sideways" to a differently numbered earth are through Otherworld, through the Superflow, using AIM's Auger, through the Nexus, and using backdoors like the darkforce dimension. I'm assuming the spider verse storyline had a mechanism, plus the Exiles had the Tallus, Thor, Dr. Strange ,Miss America Chavez, various cosmics and elders, and Quasar have it in their power set.
Someone mentioned the infinity gems/gauntlet. Their limitation is that they are stuck in their universe of origin.