A magma cube in the Nether (via screenshots)
Magma Cubes can spawn anywhere in the nether at all light levels as long as the space they are trying to spawn in is clear of liquid and solid obstruction. Since they are inherently fireproof, they can even spawn on top of magma blocks. Their size is affected by the set regional difficulty.
They have different spawning patterns across different biomes. In Nether wastes, they are uncommon but will spawn in groups of 1-4. They are more common in Nether fortresses in groups of 4, and are even more common in basalt deltas in groups of 2-5.
Magma Cubes in a Fortress (Via Screenshot)
Magma cubes move around by hopping every 2-6 seconds (40 to 120 ticks). They will look for players within 16 blocks. If they don’t spot any enemies they change their direction and keep hopping around. They are immune to fall damage and fire damage. They are not slowed down by lava and they can swim and leap above its surface. Their jump height is dictated by their size, and their forward movement speed is twice that of most other mobs in the game. They don't have any idle sounds but you will hear a sound whenever they jump and land. When a large magma cube is killed, it will split into 2-4 smaller magma cubes.
Magma Cubes attack the player by jumping and landing on them, causing damage when touched anywhere. They are quite unpredictable as their jump rate is random, making them harder to fight. They also attack quite faster than other mobs. Zombies attack once per second but magma cubes attack twice per second and their attacks can cause significant damage to the player. A large Magma Cube can deal 12 damage a second on normal difficulty. Tiny magma cubes will also deal damage to players.
An aggravated magma cube jumping toward a player (Via Screenshots)
Since there are three sizes of magma cubes, they all have different drop rates for items -
Magma Cubes are important if you are looking for some magma cream since they are one of only two ways a player can get magma cream. These mobs are quite dangerous as well, so be cautious around them, as they are quick and can deal a lot of damage in a short period of time.
A. Yes, they will take damage and eventually die when placed in water. However, they do not take damage from rain.
A. Yes, Magma Cubes are exclusive to the nether and cannot spawn naturally in the overworld.
A. No, Magma Cubes cannot be bred.