No, there isn’t necessarily 1 MiB of memory on a PC XT or AT’s motherboard. In fact the original PC (5150) only officially supported up to 64 KiB of memory, 256 KiB in later models; the original XT (5160) only supported 256 KiB, later models supported up to 640 KiB. Additional memory could be installed using expansion cards.
Expansion cards, along with the system’s memory controller, sit on the system bus, which includes a number of address lines. When the CPU wants to read from or write to memory, it puts the address it’s interested in on the bus, and asserts the appropriate state using other pins; any interested party on the bus can respond appropriately to handle the read or write.
This means the address space can cover a variety of different hardware. On a typical XT, you’d have up to 640 KiB of real memory, responding to the first 640 KiB’s worth of addresses; then one or two graphics adapters, then expansion cards, then the system ROM. The fact that addresses map to something all the way up to 1 MiB on a standard PC or XT doesn’t mean that it has 1 MiB of physical memory. The AT added memory up to 16 MiB, but it would typically have a hole from 640 KiB to 1 MiB (to leave room for expansion cards and ROM).
See Who decides what is the memory address that the CGA video buffer will be mapped to? for some related information.