They are still assigned under the hood by the linker. The magic is in the atari-cart.cfg
file which tells the linker where the data segments go. For cartridges .SEGMENT "DATA"
starts at $2000.
As for the actual register: The 6502 has them on the chip itself. They have no memory address at all.
That's great to know. I'll take a look at the 6502 datasheet. Time for me to learn about other 8-bit microprocessors.