Block refers to options for the currently active block as you’re editing. Document refers to things relevant to the page/post you’re editing (template, parent page, etc.)
You can get to an HTML view by clicking the 3 dots below the “Howdy, your name”.
See https://i.imgur.com/3olq9DC.png
See https://i.imgur.com/WMjLNut.png
See https://i.imgur.com/tyXmwUU.png
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