I’m new to Shopify and starting to wrap my head around how the template system works, but I can’t work out where the below section is defined in Dawn 9.0
The “Email signup” section is newsletter.liquid and located in sections folder.
Generally, All defined sections which save in sections folder can be used in all template page, but if section schema has presets and presets.name property, then it will list on the “Add Section” dropdown list, and vice versa. And, enabled_on/disabled_on can also control the section whether list on the the “Add Section” dropdown list or not. you can get more details at : https://shopify.dev/docs/themes/architecture/sections/section-schema#presets