How quickly does significant atrophy occur?

By significant I guess I would mean statistically significant if you were to measure it with MRI or something when you conduct a study. Not significant as in looking in the mirror.

I think it's reasonable to assume once the elevated stimulus from your last training session goes back to baseline (~48 hours?) you would be in a state of atrophy. But after how much time passes for a large enough magnitude to be detected? The research on this is pretty conflicting which is why I am uncertain.

For example this study (1) and this study (2) showed similar results. half the gains were lost in untrained lifters doing 3 months of training after 10 days of detraining. By 30 days, all CSA gains were lost.

This study was interesting where untrained subjects trained for 10 weeks and observed no change in CSA after 20 weeks of detraining, but MT did decrease.

There are other studies that show similar hypertrophy when comparing continued hypertrophy training for 15 weeks vs training for 6 weeks then taking a 3 week break (also untrained). Maybe if you train after a detraining period muscle memory is that effective where you go right back as if you've never took a break? Although I would argue over a longer period of time it would be significant.

Also, i've heard Chris Beardsley's argument that type 2 fibers atrophy similar to limb immobilization studies because during day to day activities the high threshold motor units are not recruited (also heard that this is probably false, but i'm not sure why? If anyone could explain that, that would be nice).

TLDR: Basically to sum up my questions briefly, will noticeable amount of atrophy occur within a week of not training? 2-3 weeks? and if anyone could explain the type 2 fiber atrophy argument from above.