Apple has acknowledged user complaints that iPhone 15 and 15 Pro phones are overheating, reports Forbes, but said that contrary to speculation, it has nothing to do with the phone’s hardware design. Forbes noted an update to Instagram has already rolled out with version 302, released September 27th, to address some of the issues.
We can never know exactly. For me I always think about the (incidental) complexity of these huge apps like Instagram.
Somebody mentioned the phone overheating when watching Reels - those short videos. Here's a made-up example (but I've written some software for video streaming services)…
Those videos are pretty short, and some people skip the clip even after less than 1 second. Instagram want that next video to be playing instantly (gotta get that dopamine hit ASAP!). A strategy you could take is have the app load the next, say, 5 possible videos in the background before you've even seen them. When the user swipes, that video is already playing. To make this even faster we could execute some recommendation decisions on-device rather than on some servers (over a relatively much slower 4G connection).
With all this complexity comes greater chance of some unexpected behaviour. Instead of loading 5 videos, maybe we accidentally load 100 and never clean up the old ones. Maybe after an OS update we need to change the way we mark a task as low priority.