What are the puzzle pieces at the top for?

Reminds me of a similar looking UI contraption I saw recently: https://staticdelivery.nexusmods.com/mods/1704/images/921/92... (the multicolored database icons at the top)

Quickly-running plugins. I haven't used them, but it seems like a important aspect of the editor. Here's the plugin preferences window: https://i.imgur.com/HAtJ5ej.png

Thank you, I was trying to figure out if it was a WYSIWYG editor or what.

It appears to be largely equivalent to the Calibre editor.

It doesn't look that different, at a glance, than the e-book editor included in Calibre.

"When I said I couldn’t work as the sole developer of Sigil people went overboard and acted like the sky was falling. Kovid Goyal from calibre stepped in to create Sigil’s Spiritual Successor which is an EPUB editor as part of calibre."

https://sigil-ebook.com/about/