This is not just about dictionaries, but also about lists.
Both have internal representations that respond to blocks like 'select item from list' that don't work on the external representations.
I can't show you the internal representations without a hex dump of RAM.
A book on data structures would give you an idea on the internal structure.
Operationally, a responseContent from a Web request is going to arrive as text.
If it is the external representation of a structure, it needs to pass thru the web block DecodeJSONAsDictionary to creat an internal structure that will respond to a list or dictionary block.
For that event block of yours, I used the Blocks Editor browser extension set to FIND the global variable that received the responseContent, and it got fed immediately into SELECT list blocks without the required decode.