Re those 2 last videos, seems like something is eating that CPU and it's constantly in Turbo Boost (?), so without activity monitor open, it's hard to say what the issue is.
As for external monitors, I read that even the 2015 15 with a dGPU gets warm just with a 1080p monitor attached (unlike 13'). You are using 4k with this model... Still, I think the issue here is mainly not the 4k monitor, but something to do with CPU load. Get it out of the case if there's one. Also as a test, you could try disabling Turbo Boost with some app.
What's the temps without Affinity photo, just idle with 4k monitor? If this program has a glitch that eats 100% of your CPU, then with other things considered, everyone would be getting this problem with ntb model.
Edit: the screen with the BBC iPlayer makes no sense. It shows fans at full throttle while the CPU and GPU temps are at 74? It shouldn't be running at full speed. Maybe some other software is setting the fan speed? I'm pretty sure the laptop wasn't designed to go to 7200 rpm at such temps.