How can I make HTTP request to each other?
E.g:
From my project application A, how can I do a request into a Project application B? Such as HTTP GET on http://somehost/projectB
Two separate Platform.sh projects are, from an application point of view, entirely independent. You can send HTTP requests from one project to another using whatever HTTP client you prefer; the fact that both are hosted on Platform.sh is an implementation detail.
Note that means that only requests that are routable HTTP(S) as defined by the target project’s routes.yaml will go through. There is no way to directly access a backend service (database or similar) in another project, unless it speaks HTTP and is exposed in routes.yaml.
(Exposing backend services directly on the web is strongly discouraged for security reasons, as such applications often do not have any strong security protocols in place and depend on being not-public.)