1. Entity models for the bosses are done using a (dated) generator that converts bedrock format models and animations to Java models and functions for armor stands. It is now better to do so using OBJMC and item displays (as I did here)

2. Death messages are done by disabling death messages, killing the player, running a tell raw command, and re-enabling death messages, however, this can be more cleanly done in modern versions of Minecraft through the /damage command

3. Music discs are done by having an advancement that triggers when the player uses music disc 11 on a jukebox with specific nbt, the advancement reward runs a function instantly revoking the advancement and raycasting as the player to the jukebox, it then stops the default music disc sound and plays the custom music.

4. Bucket of lobsters are done similarly just detecting when a player right clicks on a wandering villager with specific nbt with a bucket. The lobster is teleported far down and the player is given a bucket that respawns the same lobster when used.

5. Exploration maps are really complicated, It's better to look at the loot table JSON file for it.

6. The lobster's death loot table has a condition for what block the lobster is standing on / what damaged it.

Eventually

Does NOT work with 1.19.3, works with other datapacks with resourcepacks just use this website to merge them.