Changes from PoE 1

In PoE 1, Essences could be used to Path of Exile 2 Currency reroll rare items into new rare ones with a guaranteed modifier.
In PoE 2, it appears you can only upgrade an item one tier, and add one specific modifier.
PoE 2 will also introduce Essences that upgrade a Normal item into a Magic item.
This allows you to get two desired modifiers on a Rare item when starting from Normal.

PoE 2 Atlas Progression
Now that we have a brief understanding of the endgame Atlas, let's cover how to actually progress it. As you might expect, completing maps will let you traverse further and further, discovering more special encounters.

To complete a map: Defeat all Rare & Unique Monsters in the map.

As you progress the Atlas and defeat higher tier maps, you'll unlock Atlas Passive Skill Points. These will be used on the Atlas Skill Tree. These points allow you to increase the danger of your maps, which in turn increases reward.
You can add more monsters, more rares, strongboxes, essences, etc. As time passes, players will find strategies that optimize the Atlas Tree, but the Tree can also be very personal.
Allocate the passives/mechanics that you enjoy, or what you character can handle.
Towers

Another important Atlas mechanic are Towers. These are locations on the Map that allow you to scout further into the Atlas, letting you plan your journey.
Clear the Tower to unveil a larger region of your Atlas. You will also use Towers to insert "Precursor Tablets." We'll cover more on this below in the other Endgame systems.
More Atlas Details
The Atlas will have different biomes, with many Maps restricted to certain ones.
You can encounter cities with special drops.
Realmgates will be used to activate other endgame encounters. (More on this below)
Reliquary Vaults will grant a random shiny item.
Unique Maps will be found on the Atlas rather than as drops.
The Seer will occasionally appear in Maps, giving you a free Unique item.
You can find Hideouts. Clearing the monsters inside will allow you to set up base there.
Some areas will be Corrupted, adding dangerous modifiers to Path of Exile 2 Currency for sale Maps.
In these maps, slaying monsters in close proximity will spawn more demonic monsters.