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Hi
The term "bounded context" belongs to the domain layer. It is a subset of the ubiquitous language of the domain, and it helps in breaking down a complex domain.
The UI is not part of the domain model, but rather some graphical manifestation of it. It is a more technical concern of the system. So the easy answer to your question is that the UI does not live in any BC because the UI is not part of the domain model.
In a SOA system, there is more or less a mapping between bounded contexts and services. A service is the top-to-bottom entity which implements a specific bounded context. It contains both the domain model layer and also the rest of the technical aspects such as DB schemas, web backends and user interfaces.
I will allow myself to rephrase the question as to what service does the UI of the page belong to?
As you correctly pointed out, in many SOA systems, like in your Amazon.com example, webpages are actually composed of multiple UI components. So each UI component which is responsible for a particular concern of the domain (BC) is implemented as part of the appropriate service (e.g. the shipping UI component lives in the shipping service which is responsible for implementing the shipping BC).
So what we're left with is a piece of UI that defines the layout of the components that will compose the page. This layout definition would probably live in its own service, whose responsibility is to, well, define the layout of webpages.
Udi Dahan, in his SOA principles course, calls this service the Branding service. This service will contain those UI layouts as well as common graphical definitions such as CSS styles and resources such as logos or other graphical entities.
Does that help?
Cheers,
Moran