The Cortex-X1 is the result of a very specific collaboration between ARM and its partners, and ARM’s not saying which phone will get it.

ARM has announced the next generation of smartphone processors, set to deliver up to 20 percent or greater performance than the prior generation. It includes something special: a new Cortex-X1 design, an optimized version designed for “ultimate performance.”
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https://www.pcworld.com/article/3544909/arm-announces-smartphone-chips-for-2021-cortex-mali-ethos-cortex-x1.html