It works fine. The processes get a fenced off piece of memory and are allowed to portion it out to themselves any way they prefer. Syscalls are designed to be mostly neutral about how the app wants to proceed.

On a system with an MMU, applications have their own address space that the allocator draws from; pages need to be centrally allocated, but individual allocations can be handled in userspace

Windows NT (NT - Windows 11) keeps applications in their own private memory space by design. Windows 1,2,3,95,98,Me didn't do this which is why it was more prone to blue screens.