Damaging temperatures are generally between 90-100°C. Throttling can happen at lower temps and those are usually CPU dependent (and cooler dependent). Also CPUs with less cores handle thermal throttling worse as they have less cores to fallback on if one overheats.
As a sometime hardware engineer, my rule of...er...thumb was that if it was difficult to keep your finger on the device for 5 seconds then it was overheating.
Damage to devices certainly doesn't happen at 100C but you might be talking about case temp under some assumptions about how that relates to junction temp for some specific setup. In general its junction temp that decides if the device suffers permanent damage.
Well I guess not damage, but most mobos have an overheat prevention mechanism that will shut the system down at those temps. Can't really use your CPU over 100°C if your motherboard shuts the system off.
Sure you can override these in the BIOS but then you're risking damaging the VRMs which might be rated at lower temps.
It's very dependent on the device. Enterprise/networking-class chips I work with are usually rated for long-term reliability up to 125C, more recently that's come down a little bit (particularly regarding memories) but still significantly over 100C.
