Can the url be manipulated within processRequest? [...] If not, what is the reason?
In a general context, in the scenario of f(g(x)), f does not have access to x directly, as it can only operate on the output provided by g. The return value from g may not necessarily contain any information about the input x. Consider g = u => 42 as an example where the function does not preserve details about its input.
However, if the return value includes the input information, such as in the case of g = u => u (the identity function), then f would have knowledge of x since it receives this exact value. Yet, this requires f to possess specific awareness of the input.
Given that HttpClient#get produces an Observable as its result, the situation differs here. To grasp this concept intuitively, consider that the url pertains to the mechanics of HttpClient, while Observable originates from RxJs and does not pertain to Angular specifically. This distinction becomes apparent when examining the type of Observable.
Is it feasible to alter the url within processRequest? [...] If not, why is this restriction in place?
It may seem obvious now, but modifying something necessitates the ability to access it first.