Earlier there was a rumour that Microsoft was planning to switch to ARM architecture in their upcoming Xbox handheld, which is not the one that lands via a partnership with Asus later this year, but later.
However, this custom-designed handheld does not appear to use ARM, but instead a custom-designed APU from AMD, according to well-established insider KeplerL2 in a post on NeoGAF:
They have a custom low power APU design with AMD for the "real" Xbox handheld", he writes.
This "real" handheld is the one Microsoft designs and builds entirely in-house, but we'll reportedly see parts of that experience via the upcoming Xbox version of Asus' ROG Ally 2, which apparently offers a separate Xbox interface via a dedicated button.