I had access last week but not anymore. The issue is that at low levels it’s inaccurate. Eg they expose it to 25 dB and it measures 30 dB. I cannot get mine to go below 29.5 dB. These apps are all mainly designed to tell you if you’re exposed to too high noise levels.Odd. On my phone (13 Pro) the NIOSH app goes down to around 27 at night in my office. It does jump up half a dB or so when my iMac Pro is running, even when I can't hear it myself. The accuracy study that I saw abstracted (The Accuracy of iPhone Applications to Monitor Environmental Noise Levels) rated the NIOSH app as the best across a wide spectrum of noise levels. Perhaps some issues at low noise levels are buried there (I didn't have access to the original paper) or it may depend on the phone.
Second - as you mention - it is very likely that each phone will have different mics or just different amounts of dirt from your pocket on them.
They can at best be used relatively. Measure noise before/after turning on the Mac. I did and there’s around 0-0.5 dB difference on my 2020 iMac, which I cannot hear at idle.
Another proof of how useless these apps are: my audio monitors are making a distinct hiss. The noise measurement apps will not register that even though it’s clearly audible.