https://wccftech.com/xbox-series-x-developer-kit-teardown-unveils-40-gb-gddr6-memory-features-20-samsung-dies-running-at-14-gbps/

GamersNexus has confirmed within a teardown video that the Xbox Series X Devkits feature 40 GB of GDDR6 memory onboard the motherboard.

Currently, the Microsoft Xbox Series X consoles shipping to gamers around the globe feature 16 GB of complete system memory. While the memory is all GDDR6, it is partitioned into two segments, a faster 10 GB segment which operates at 560 GB/s (14 Gbps @ 320-bit), and a slower 6 GB segment which operates at 336 GB/s (14 Gbps @ 192-bit). That's the configuration used across all of the XSX consoles but the developer kits feature more than twice the memory as the top-tier console.