Bosses
You can find Fire Golems atop randomly generating towers in the Overworld.
The Corrupted Golem must be summoned. The Corrupted Golem is spawned by placing 4 Iron Blocks a T shape, and putting 3 Bone Blocks on top of the three upper blocks. The last block placed must be one of the three blocks of bone. This process is similar to summoning the Wither.
The Wither must be summoned. The Wither is spawned by placing 4 blocks of Soul Sand and/or Soul Soil in a T shape, and putting 3 Wither Skeleton Skulls on top of the three upper blocks. The last block placed must be one of the three skulls.
The Blackstone Golem must be summoned. Get 4 Gilded Blackstone Shards by bartering with piglins or find them in the nether bastions. Then build 4 Chiseled Polished Blackstone Holder by putting Chiseled Polished Blackstone into a stone cutter. Next, arrange blackstone blocks in a 9x9 square, and place a blockstone holder outside the square in the center of each edge. Lastly, place the shards into the holders to summon the boss.
You can find the Ender Dragon in The End. It will automatically appear if it is the first time The End has been accessed. Players can re-summon the Ender dragon once the fight is over by placing four end crystals on the edges of the exit portal, one on each side. When the dragon is re-summoned, a series of explosions resets the obsidian pillars, iron bars, and end crystals. The top of each pillar appears to explode, destroying any player-placed blocks. If the player does not pick up the dragon egg and starts to re-summon the dragon, the dragon egg disappears.