Haven’t played with the Garmin body temp app. Don’t know how they estimate core temp from HR, but I can’t imagine it’s super accurate at an individual level.
Garmin has the Tempe sensor which I’ve used for skin-temp readings indoors. Hook it to my HR band on the side of my chest. Can definitely see the massive difference from fan vs no-fan (like 32deg vs 26deg readings) and different clothing layers. But unfortunately it won’t connect to a Garmin that already has on-board temperature sensor, like most of the Edge units, so I haven’t used it out on the road.
Skin temp isn’t core temp, but it’s one part of the temperature gradient that’s relevant for how performance is limited by thermoregulation. Core temp basically reaches a limit that the body is unwilling to exceed, so performance and further heat generation will be throttled. A bigger gradient (greater difference in temp) from core to skin and the surrounding environment allows the body to shed heat faster, therefore perform more work at a higher rate of heat generation to match the rate of cooling.
Hence why fans to increase skin cooling are so critical for indoor performance.