What decides where on the heap memory is allocated?

The memory is managed by the OS. So the answer depends on the OS/Plattform that is used. The C++ specification does not specify how memory on a lower level is allocated/freed, it specifies it in from of the lifetime.

While multi-user desktop/server/phone OS (like Windows, Linux, macOS, Android, …) have similar ways to how memory is managed, it could be completely different on embedded systems.

What is in control of choosing where on the heap memory is stored and how does it choose that?

Its the OS that is responsible for that. How exactly depends - as already said - on the OS. The OS could also be a thin layer in the form of a combination of the runtime library and minimal OS like includeos

Does this mean that the heap is shared between all processes?

Depends on the point of view. The address space is - for multiuser systems - in general not shared between processes. The OS ensures that one process cannot access memory of another process, which is ensured through virtual address spaces. But the OS can distribute the whole RAM among all processes.

For embedded systems, it could even be the case, that each process has a fixed amount a preallocated memory - that is not shared between processes - and with no way to allocated new memory or free memory. And then it is up to the developer to manage that preallocated memory by themselves by providing custom allocators to the objects of the stdlib, and to construct in allocated storage.

I want to learn more about memory fragmentation

There are two ways of fragmentation. The one is given by the memory addresses exposed by the OS to the C++ runtime. And the one on the hardware/OS side (which could be the same for embedded system) . How and in which form the memory might be fragmented organized by the OS can't be determined using the function provided by the stdlib. And how the fragmentation of the address spaces of the process behaves, depends again on the os and the also on the used stdlib.

None of these details are specified in the C++ specification standard. Each C++ implementation is free to implement these details in whichever way that works for it, as long as the end result is agreeable with the standard.

Each C++ compiler, and operating system implements these low level details in its own unique way. There is no specific answer to these questions that apply to every C++ compiler and every operating system. Over time, a lot of research has went into profiling and optimizing memory allocation and deallocation algorithms for a typical C++ application, and there are some tailored C++ implementation that offer alternative memory allocation algorithm that each application will choose, that it thinks will work best for it. Of course, none of this is covered by the C++ standard.

Of course, all memory in your computer must be shared between all processes that are running on it, and your operating system is responsible for divvying it up and parceling it out to all the processes, when they request more memory. All that "returning memory to the OS" means is that a process's memory allocator has determined that it no longer needs a sufficiently large continuous memory range, that was used before but not any more, and notifies the operating system that it no longer uses it and it can be reassigned to another process.

What decides where on the heap memory is allocated?

From the perspective of a C++ programmer: It is decided by the implementation (of the C++ language).

From the perspective of a C++ standard library implementer (as an example of what may hypothetically be true for some implementation): It is decided by malloc which is part of the C standard library.

From the perspective of malloc implementer (as an example of what may hypothetically be true for some implementation): The location of heap in general is decided by the operating system (for example, on Linux systems it might be whatever address is returned by sbrk). The location of any individual allocation is up to the implementer to decide as long as they stay within the limitations established by the operating system and the specification of the language.

Note that heap memory is called "free store" in C++. I think this is to avoid confusion with the heap data structure which is unrelated.

I understand what the stack is

Note that there is no such thing as "stack memory" in the C++ language. The fact that C++ implementations store automatic variables in such manner is an implementation detail.

The heap is indeed shared between processes, but in C++ the delete keyword does not return the memory to the operating system, but keeps it to reuse later on. The location of the allocated memory is dependent on how much memory you want to access, there has to be enough space and how the OS handles memory allocations, it can be one on first, best and worst hit (Read more on that topic on google). The name RAM basically tells you where to search for your memory :D It is however possible to get the same memory location when you have a small program and restart it multiple times.