Hey there and welcome to the Community, @BEDO 
We’ve described how to set a specific Blog category by default in this article. Give it a try and let me know how it worked 
Regarding the Code block option, it lets you attach the code snippets right in the posts, but they won’t be transformed into widgets (like you’ve done with the Appointment button in the title).
If you’d like to embed the other widgets into the Blog posts, please upvote this idea - Embed other widgets into Blog