How unallocated memory space is represented?
Each page of RAM is typically a 4k block, with a set of privileges on it such as read, write, or execute. Broadly speaking, the memory manager for the operating system will allocate a bunch of pages for OS related storage, and when a program launches, it will allocate initial RAM, and as more items are allocated, the program will ask the operating system for more RAM. 

From the program's point of view, the RAM is all contiguous. However, under the hood, the operating system is mapping the various blocks of system RAM, and each program's RAM.

Physically, the RAM is broken up into blocks, which are essentially unorganized, but the operating system itself has a table mapping what block belongs to each program.