This is a developer API not meant to be used by the end-users. Replacing Bukkit/Forge/Sponge with this will **not** work since we do not implement any of their APIs.
Minecraft has evolved a lot since its release, most of the servers today do not take advantage of vanilla features and even have to struggle because of them. Our target audience is those who want to make a completely different server compared to vanilla Minecraft such as survival or creative building.
The goal is to offer more performance for those who need it. Minecraft being single-threaded is the biggest problem for them.
It is our major concept, worlds are great for survival with friends, but when it scales up it can become unmanageable. The best examples can be found in Skyblock or minigames, not being able to separate each part properly and being forced to save everything in files, not to say the overhead caused by unnecessary data contained in them. Instances are a lightweight solution to it, being able to have every chunk in memory only, copying and sending it to another player in no time, making your serializer and much more...
Instances also come with performance benefits, unlike some others which will be fully single-threaded or maybe using one thread per world we are using a set number of threads (pool) to manage all chunks independently from instances, meaning using more CPU power.
The terms "passive" or "aggressive" monsters do not exist, nobody forbid you from making a flying chicken rushing into any players coming too close, doing so with NMS is a real mess because of obfuscation and the large inheritance.
It is a field where Minecraft evolved a lot, inventories are now used a lot as client<->server interface with clickable items and callback, we support it natively without the need of programming your solution.
Commands are the simplest way of communication between clients and server. Since 1.13 Minecraft has incorporated a new library denominated "Brigadier", we then integrated an API meant to use the full potential of args types.
