Can’t get header.html to show in my index.html

Hi,

Brand new to WordPress block development and I am trying to set up a "default" theme using theme.json, index.html and header.html, to learn how everything works. One thing I can't seem to grasp is, that even though I have a header and try to load it through the index.html file (see images below), the header won't load. I thought the whole idea of using a "global" setup for your theme is to have it as the default when loading the file.

Structure is correct and was setup by the Create Block Theme addon.

https://preview.redd.it/h03b97nnd6ld1.png?width=340&format=png&auto=webp&s=3e9e73776446db7118986c87abb01a69cb51b5e3

So my question is if I need to go WordPress->Editor->Index, and add the header in there, and also have the code inside header.html? This seems to be the only way to get it to show. But I thought the whole idea was that I could leave these files blank, and the design would load from my local files instead? But this isn't working.

theme.json

index.html

header.html

 

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