@prw, Thanks for rewriting your setup and filling in the blanks. Much clearer this way!
As quoted before:
Blocks are used for inheritance and act as placeholders and replacements at the same time.
Blocks are only used for inheritance. Code embedded in ‘block’ tags can (but don’t have to) be overridden by inheriting (child) templates. If overridden, the content of the block tag is replaced (‘replacements’) , else the existing content (‘placeholder’) will be rendered.
Blocks are not needed if the page is not inherited. This means, the ‘block’ tags in template ‘pagebuild.html.twig’ serve no use and could be left out.
The content of the blocks are rendered normally even if not inherited.
Note: Because ‘pagebuild.html.twig’ is not inherited by ‘modular-extension.html.twig’, the ‘content’ block in ‘modular-extension.html.twig’ will override nothing. Twig is very forgiving and will not raise an error.
As a side note: Your folder structure below ‘02._main’ looks odd, both with respect to naming and existance of subfolders. Since ‘pagebuild.md’ is not a modular page I wonder what function they serve…