The Overworld
The Overworld is the default: day, night, weather, villages, caves, oceans, and the deep dark under the deepslate. Since the Caves & Cliffs update the ground runs from Y −64 to Y 320, so “dig straight down” is an even worse idea than it was in 2011.
Biomes decide trees, mobs, structures and whether you should have brought a boat. Ancient cities sit in the deep dark. Woodland mansions and trial chambers are the mid-game “I saw it on the map and I regret it” buildings. A compass points at your original spawn, not at your bed.
The Nether
A Nether portal is a rectangle of obsidian — at least 4 wide and 5 tall, corners optional — with fire inside. A flint and steel is the usual match. Step through and you are in a roofed cavern: lava seas, fortresses, bastions, warped and crimson forests, basalt deltas, soulsand valleys.
Distance is the trick. One block in the Nether is eight blocks in the Overworld. Walk 100 blocks in the Nether and you come out 800 blocks from the matching Overworld portal. That is why ice highways and boat roads live there. Divide Overworld X and Z by 8 to place a portal; ignore Y. Build the return portal and light it, or the game will guess and link you somewhere annoying.
You go for blaze rods (fuel for brewing and Eyes of Ender), nether wart, quartz, glowstone, and ancient debris, which smelts toward netherite. Piglins trade if you wear gold and do not open their chests in front of them. The bedrock ceiling is real. Breaking onto the roof is a Java habit that many servers ban. See Realms and small SMPs.
The End
Eyes of Ender, thrown in the Overworld, drift toward the nearest stronghold. Follow them, dig in, and find the portal frame. Fill the empty slots with more eyes — up to twelve — and the starfield opens.
The central island holds the Ender Dragon and the crystals that heal her. Bring beds or a slow bow plan, knock out the crystals, then the dragon. The exit portal that appears starts the poem and the credits. That is the closest the game has to an ending. Most players treat it as the door to flight, not the credits they sit through.
Gateway portals around the island lead to the outer End: chorus plants, end cities, and ships with an elytra in the item frame. A pair of elytra plus fireworks is the late-game way to travel. Shulker shells, dropped by the mobs that guard those cities, become boxes that keep their contents when broken. That is the inventory upgrade people mean when they say the End “changes the game”.
How this sits in a normal playthrough is the survival loop. The rods and wart you came for are on the crafting and brewing page.