How to write keyboard dll using KbdLayerDescriptor character?

Searching for an example source code was insufficient and I could not find the official documentation about the sign KbdLayerDescriptor

. So I have a few more questions:

• What is the purpose of a ligature table, or more precisely, how does it work. Is it for writing pre-written characters? If it is not, does it mean to automatically insert the ZERO WIDTH JOINER character or just write a few characters without a ligature?
• Can three or more shift states be defined using the numeric keypad keys?
• I saw what needed to be defined KBD_TYPE
  
  . What is the purpose of each integer value?
• Can I use Unicode values ​​larger than 16 bits, such as math 𝚤
  
  ?
• I've seen the keyboard layout use [HKLM\SYSTEM\CurrentControlSet\Control\Keyboard Layout\DosKeybCodes]
  
  and [HKLM\SYSTEM\CurrentControlSet\Control\Keyboard Layouts]
  
  , but it seems that these are not the only registry keys that need to be executed in order to register the system keyboard. So what are the required registry keys to install the keyboard layout throughout the system?