Official servers are cancer, people are just toxic, this applies to just about all games, if someone finds a way to cheat, then they will. Wildcard has been cracking down on cheaters using exploits like meshing etc, but obviously people always find a way to be shit humans.
The base game has 1 map, The Island
They then added a bonus free map The Center, it’s basically just another island map, but vastly different topography.
Then Scorched Earth expansion(paid DLC) came out, it’s a pretty much full desert map.
Ragnarok was made and added as a free bonus map, it combines elements of The Island and Scorched Earth, it has deserts, redwoods, forests etc.
Aberration expansion(paid DLC) was next, the whole map is basically a giant cave system with 3 levels of danger, upper green zone, dangerous blue zone in the middle, and very dangerous and radioactive zone in the bottom, the surface is on fire during the day, you can go there during the night, but there are also dangerous terrors buried in the dirt.
Extinction expansion(paid DLC) came out after, you are basically back on earth, walking through the ruins of ancient civilizations.
Valguero was added recently as a free bonus map, it has all the elements of The Island and also a cave system from Aberration. Unlike Ragnarok, it does not have a desert region.
Genesis is the new season and paid expansion out most recently. Genesis has 1 map, but it’s separated into different biomes. This is currently part 1 that is out at this time.
Crystal Isles, new bonus free map coming soon.
Genesis part 2 will come later, not available yet, still being worked on.
In terms of purchasing the game
The base game, has the main map The Island, and all the free bonus maps are accessible.
If you want Scorched Earth, Aberration, Extinction, you need to buy them either individually or cheaper through the first season pass, which includes all 3 dlc expansions, plus some season pass exclusive skins.
Ark: Survival Evolved Explorers Edition includes the base game and first season pass.
Genesis Season Pass gives you access to Genesis Part 1(currently out), and Genesis Part 2(coming soon)
Keep in mind that each paid expansion will also through giving you access to those maps, give you access to new engrams that are learned on those maps, like being able to make a chainsaw(SE), glider attachment and climbing picks(AB), Mek suit(EX), mining drill and ocean platforms(GEN). And lots more, I am merely listing a few examples.