Table from compressed binary encoded text

I have taken over a Power BI report from someone who has now left (so I can't ask them questions), and I came across the attached query which I am trying to wrap my head around. I want to understand how it works and why "2023" isn't in the table.

I see that it is doing a binary encoding in base64 of that string (why that string?), and then decompressing it (why encode and then decompress?), then returning the results of that json document (why is it a json document?), and then creating a table by parsing that list. The "let _t = ..." I guess is defining the column or something, but I don't really understand any of the syntax in that bit. Overall, why does this work, and how do I get "2023" to appear?

Is it just a matter of that specific string encods and then decompresses into that specific list? And therefore a different string would give a different list?

How would I go about testing all the parts of this DAX individually?

Thank you.