Start by checking which apps are consuming the most power by going into battery settings - you might have an app that's been broken since the update and is making the processor run at full throttle. I'd say 90% of abnormal overheating cases are just an app glitching, not Android 15 itself. Try disabling push notifications for your most power-hungry apps (especially social media and games) and see if that makes a difference.
If it's not an app causing the problem, try something simple before digging deeper: completely restart your phone and let it cool down for a good half hour. Then launch a video or game and see how it performs. Sometimes Android 15 just needs a reboot to stabilize after the update. If it's still heating up, you can also try going into developer settings (tap the build number seven times) and check if you've enabled something that boosts performance and uses more power. Stuff like ray tracing or certain GPU settings can really drive the temperature up.
As for whether others have the same issue, yeah some people are complaining but it's far from massive on the Pixel 8. It could also be related to external temperature - in the middle of summer like this, your phone's already starting from a higher baseline. A worst-case scenario is that your battery's aging, but on a Pixel 8 that's unlikely unless you've had it since launch. If after all this it keeps heating up badly during normal stuff like watching YouTube, it might be worth hitting up Google support or checking your warranty.