Theme settings are all stored in settings/settings_data.json
Schema definitions for theme settings are stored in settings/settings_schema.json
However, for each section it’s schema is defined in respective liquid file under sections.
And if you want to inject your setting into section settings, you need to add it’s definition to the schema part into respective section file for it to be visible in your liquid.
However, if you inject your variable into theme settings it will be visible even without declaration.
Here are the knowledge base links for your reference: https://shopify.dev/docs/themes/sections and https://shopify.dev/docs/themes/settings