I understand almost what the Kernel does:
Central part of operating system and any interaction with the computer hardware is done via a system call.
How does this work and what does it mean with layers

You have user and kernel layer.
User layer's functions cannot alter the basic variables (OS Core), and have limited access to data and memory.
Kernel layer is base of OS, and can alter all data, and have unlimited access to all resuorces.
Kernel layer works with system processes/threads, memory etc.

Simply: User mode (that is what you can alter)
Kernel mode (hidden in background)

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