This page of the guide to Ratchet & Clank Rift Apart explains how jumping between dimensions works. You will learn exactly what this mechanic is and if you can access it at any point during the game.
The following description contains minor spoilers about the general premise of the game. You won't find any major plot spoilers or information on how the adventure will end.
Different dimensions - general information
There's a device related to traveling between different dimensions, and it's called the Dimensionator. At the very beginning of the game, it falls into the hands of the sinister Doctor Nefarious. In the later stages of the campaign, the characters strive to obtain the blueprints that will allow them to make another Dimensionator. Thanks to unlocking other dimensions, new important characters could join the game's story - Rivet (just like Ratchet, she represents the Lombax race) and Kit (in a nutshell she is a female "variant" of Clank - a machine that acts mainly as a companion).
At important moments in the story, interdimensional travel is pre-determined and you cannot influence it. The main character can, for example, fall into an interdimensional rift and thus move to a completely new place.
Thanks to the technical capabilities of the PS5, there's no loading between visits to other dimensions. The game loads a new location/another dimension instantly.
In other dimensions, you can meet alternate versions of the characters and villains known from previous installments of the Ratchet & Clank series. The events in the other dimensions may have also been different. For example, in one dimension, Nefarious gained the emperor status and became the ruler of the galaxy.
While playing the game, you can also witness scenes that show the instability of individual dimensions or the transfer of monsters from other dimensions to the world currently explored by the character you control. All of this is supposed to add variety to the gameplay.
Inter-dimensional travel on demand - is it possible?
Most of the time, the travel between dimensions is automated. This means that the dimensional shift occurs without your involvement, for example during an cutscene or a chase scene that allows you to cross interdimensional portals.
The first exception to the above rule is finding and manually "opening" the Pocket Dimensions. These are small locations treated as separate dimensions. When you go through the rift, you will find yourself in a completely new environment. These places are obstacle courses where you have to avoid traps or complete environmental puzzles. The aim of this is to find the hidden unique armor pieces (a type of collectibles).
The other exception are Rifts. These are large purple crystals that you can find on Blizar Prime.
Hitting them with a melee weapon allows you to move between two variations of the same location. In the case of the aforementioned Blizar Prime, it can be seen as two timelines - in one of them, the location is still being mined and you can meet some friendly NPCs, while in the other version, a great cataclysm has already taken place and the place is ruined and abandoned.
Small rifts are also associated with the different dimensions to some extent, and you can find them practically throughout the whole game. They allow to bend the time and space - Ratchet and Rivet can "latch" onto them to them and, for example, avoid chasms, escape enemies or generally reach places that are difficult to access. Using small rifts, however, does not cause the character to move to another dimension. The character remains in the same reality.