Hey there, Carlos
Current "Code" button works as both inline
(when selecting less than one line) and
multiline code
(when selecting more than 1 line).
Indeed, there are no heading buttons other than H2 as it's already quite hard to fit all the buttons. As a compromise, H2 header is just the style that most people wish for when applying headers.
Of course it could be cool if there were dropdown options for headers buttons: for example increase/decrease header level on selected text (in marktext it's called "promote/demote headers"). Although, it's never been a priority, I think it could be a good first "issue" for new Joplin contributors.
More on that was summarized here
In any case, welcome to the community