Short answer: No.
Long answer: Noooooooooooooooooooooooooooooooooooo. You've made a fundamental misunderstanding about the nature of the early IBM PC hardware, and all those hours of puzzling could have been solved by, well... going back and reading up on it.
The PC and XT mainboards could address a maximum memory range of 1 megabyte, as that's all that the lines coming out of the processor could span. But, the architecture had to allow for various other things in the memory map: a not inconsiderable amount of onboard ROM (the early machines being BASIC-centred, as neither a disk drive nor a DOS could be assume as present), plus any number of expansion cards (the machine was made to be hugely expandable - you could even buy a secondary Expansion Chassis box with a load of extra slots and its own power supply) acting as memory-mapped IO devices, often with their own RAM and ROM that needed access ranges outside of the usual RAM zone.
Initially this was split 50/50, as in 1981 a half megabyte of memory was huge; the base model of the PC came with 16KB, and the deluxe model had 64KB. When power users started demanding even more, and it became clear that 384KB of address range was more than enough to fit in the usual complement of ROM and IO, a revised BIOS was issued that adjusted it to more like 5/3... that is, you could have upto 640KB of RAM without using special banking/paging drivers to access supersized cards. Hence the familiar conventional memory limit of many successive generations of DOS.
No PC/XT therefore has 1MB on the motherboard. 3/8ths of it would be wasted, and at early 80s prices, that's a LOT of money to burn for no reason. It may look like there's two equal banks on some of them, but that's an illusion; one will use more expensive, high density chips to provide 512KB, the other will use cheaper low-density ones (but the same amount, as typically they're arranged as 1-bit wide devices, thus there's 8 in a bank) for the additional 128KB. Others will make it a little more obvious by having three banks; two of 256K and one of 128K. A lot of XTs were sold with 512K on board and the user had to supply the rest.
ATs, it's anyone's guess. A lot were sold with 512K or 640K just because that's what made economic sense in the market segment and was most convenient (no need for extra drivers under DOS or whatever), and some early configurations even had less. But I doubt it would have been uncommon to find later ones with at least a full meg onboard, and/or maybe additional SIMM slots or an EMS/XMS memory card in one of the expansion slots to increase the total amount to 1MB or more with use of HIMEM or other such drivers. They could support more, natively, indeed all the way up to 16MB (though with the typical PC 640-to-1024K memory hole, and another near the top of the expanded range, a max of 15MB was more common), but a fully loaded example would be a fancy machine indeed, and expensive with it. I doubt many ever had more than 2MB, or 4 at most. Either way, just having 1MB on the mainboard would likely have been an uncommon occurrence overall, apart from a certain period, probably just before they started giving way to 386s in great numbers, when that became a briefly common standard setup.
(I actually have a 286... with 640K on the mainboard, and two SIMM slots. Populated with the only valid configuration, it's still not "one meg" - instead, it's 1152KB)
As for the video cards, etc... yeah, their onboard memory had to fit into the overall system map, but it didn't become a true part of it. You couldn't load programs into it and execute from there, as far as I'm aware. Though I'll admit a little confusion on that front myself, as ROMs that sit in the over-640KB region are entirely executable without being "shadowed" into main RAM, so, maybe? I don't think many people have tried it as a realistic thing, though, because by the point graphics cards started coming with a large enough amount of memory to make that a worthwhile tactic (and started pushing the boundaries of what would fit in the IO zone) - namely, enough to allow multiple logical graphics pages, so you could write data into a part of the adapter that wouldn't immediately show up on-screen, and pull off tricks like double buffering - actual system memory had also expanded quite considerably and it wouldn't have given quite the same boost as before. Plus reading anything over the expansion bus was pretty dang slow, hence the common tactic of "shadowing" ROM for faster execution. Plus any card with more than 64KB - e.g. VGAs, or EGAs with more than the basic, somewhat crippled memory count - didn't expose all their memory in one go, and instead had paged access via 64KB blocks and a page register, to avoid overloading the available address space (especially crucial with the PGC, XGA and other expanded-VGA cards that had 320, 384, 512KB or more...)