The WebSDK applications are usually built with the dynamicOptionsUrl (Web SDK documentation) pointing to /apps/public/public-options/options.json.
During application startup, the application accesses /apps/public/public-options/options.json to fetch the further options (like e.g. the branding) from there and applies these to the application.
The built in branding manager (Enterprise tenant - Cumulocity IoT Guides) utilizes the same functionality to apply the branding to all of the apps of the tenant and it’s subtenants.
By subscribing to the public-options app after it’s creation, it changes it’s availability to MARKET. Apps with availability MARKET are also available to subtenants, by having the public part in /apps/public/public-options/options.json. So the branding also applies to subtenants unless they have their own public-options application.