There are a few things you should now: first, the actual physical memory of your computer is pretty much like a big array: if you were writing code in assembly and were to run it with no operating system, that's exactly what you'd get.
However, with operating systems and multiple programs running at the same time, memory is no longer contiguous: instead, programs can request "pages" (blocks of memory). This is (more or less) what `malloc` does, if you've come across it. That's the key difference: in a modern operating system, you can't expect memory to be one big array, since your program might have requested more than one page of memory. In that sense, it's more like a collection of smaller arrays.
We have to do it this way so we can have memory protection (similar to file permissions - a program can decide if other programs can read one of their pages, write to it, etc) and swapping (i.e writing unused pages to non-volatile storage , like a hard drive, to free memory).
Not only that, but the linearity of physical ram is a fiction as well: in nearly all systems these days ram is made up of multiple memory modules (the MM in SIMM/DIMM), and to my knowledge, the OS is free to stitch them together in any way it sees fit.
(All of this is to say nothing of NUMA.)
However, one of the responsibilities of the OS is to hide all that messy detail from the bare-metal programmer or compiler writer and provide a simple(r) abstraction over the hardware. Thus, "(physical) memory is a big array".
On PC, BIOS configures memory controller in such way as to hide boundaries of memory modules from OS. Resulting address space still contains some holes and stuff that is not physical memory, but OS gets map of this from BIOS. Originally (on systems with 36pin SIMMs) this wasn't the case and you had to match memory modules such that they produce continuous block of addresses.
In essence the situation is pretty much same as for user space program: you get big address space and list of memory regions that are mapped and usable.
However, with operating systems and multiple programs running at the same time, memory is no longer contiguous: instead, programs can request "pages" (blocks of memory). This is (more or less) what `malloc` does, if you've come across it. That's the key difference: in a modern operating system, you can't expect memory to be one big array, since your program might have requested more than one page of memory. In that sense, it's more like a collection of smaller arrays.
We have to do it this way so we can have memory protection (similar to file permissions - a program can decide if other programs can read one of their pages, write to it, etc) and swapping (i.e writing unused pages to non-volatile storage , like a hard drive, to free memory).