Rest assured, Apple will have multiple prototypes running Ryzen, ARM and perhaps their own CPU designs. Just look at their history;
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I don't think their own SoC/CPU designs are in the "perhaps" category. Those are probably more likely than the others. iOS shares a common foundation so it is to a very large extent what they are doing anyway. Perhaps not porting some of the older drivers ( Firewire) but the core macOS and some of the mainstream bundled apps.
Ryzen (AMD) isn't some "Land Man on the Moon" project either. AMD is just as x86_64 99+% user space the same as Intel's implementation. There is only some narrow set of low level privilege mode and mild gaps on AVX implementations to deal with. intel and AMD have cross licensing on x86 instructions so there is no huge gulf here at all.
[ Apple chasing the NUMA impacts the first two iterations of Zen had? Probably not. Deep compiler optimizations on Apple's LLVM fork? Probably not. Working enough to run test? Apple can 'hackintosh' probably at least as good as folks on the outside. ]
3rd party ARM solutions? Errrr, that is the one that is a bit doubtful. Qualcomm , Samsung , Mediatek , Hisilocon/Huawei ... I'm not sure why they'd spend tons of effort helping Apple scope their processors as their are deep competitors at this point. The sever ones Ampere / Marvell/Cavium ThunderX2 in Apple Data center perhaps, but the track record of those to bet the Mac desktop farm on seems to be a stretch.
Apple putting prototype N1 reference board through the paces ( https://www.anandtech.com/show/13959/arm-announces-neoverse-n1-platform ) maybe. But highly likely lower priority than the other two. It is aimed at a different workload that higher end Mac systems are. Probably not going to significantly gap both Intel and AMD over next few years.
that video about Apple moving to Intel (from PPC) to chase performance per watt has more weight on the laptop part of the Mac line up versus the desktop one. The new Mac Pro weighs in a 1.5KW power supply. That isn't modest sipping of power. ( I'm sure over time Apple would like to do "more" with the same 1.5K budget because can't get any more out of a normal plug. But points to different parts of the line up aimed at significantly different targets. )
The overall Mac market is relatively small to 'split' into subdivisions, but Apple may go that course ( especially if they can just 'reuse' pragmatically already 'paid for' iPadOS SoC. ). Chasing the ever thinner laptop with that.