CI3 to CI4 - hardest transition points?

Personally, the following topics are of interest to me:

• Namespaces practical use in CI4 with examples
  
• Routing and more practical examples  
  
• Working with database - real app with examples queries  
  
• How to use .env in practical examples  
  
• How to use Filters in practical examples  
  
• How to use Events in practical examples 
  
• How to use Honeypot in practical examples 
  
• How to use Migrations in practical examples
  
• How to use and work Cache / DB-cache in practical examples
  
• How to create API in practical examples
  

And ofcourse i am very waiting your book (delete link)

I don't use these COMPOSER and things like that.
As for the file .env, I also remove it from the application as it is easier for me to configure in the configuration files themselves.

What I agree with is that there is no detailed (or incomplete) description of the last six items in the official documentation, starting with the filters.

P.S.: +1 for the last 6 points.