8-bit CPUs usually had a 16-bit address bus. (Motorola had a unified address bus, RAM and peripheral I/O shared the same address space, Intel chose to divide the two. In the case of Intel, the IO address limits of the 8088 and 8086 carried the limits over from the 8080 & 8085 CPUs.)
Intel's 8088 and 8086 had a 20-bit memory address bus(1MB), while Motorola's 68000 had a 24-bit address bus (16 MB). IIRC, the [80]286 jumped to a 24-bit address bus. Both later expanded to a 32-bit address bus with the [80]386 and the 68020 respectively.) With the Pentium chips, the address bus expanded to 64-bits. (I think the Motorola/IBM venture PowerPC chips also went 64-bit address bus.)
Memory available below and up to the maximum that could directly be accessed by the CPU was only limited by the supporting harwarehardware chips (chipset) and OS. Bill Gates was famous in the past for stating that nobody needed more than 640K of RAM, thus DOS never evolved to directly access more RAM. With HiMem.sys and EMM386, DOS was extended to access more "upper" memory, with EMM386 being used to directly access all available RAM. HiMem.sys had less flexibility and could basically use the extra RAM for storage.
Memory exceeding that limit required a MMU (Memory Management Unit) to break the memory into segments and map it into the addressable memory space of the CPU. It's how the CoCo 3, Commodore 128, and other 8-bit computers could access more than 64K of RAM.
More favorable now is to use virtual memory to extend past physical memory limits, albeit with the limits imposed by the OS.