In such cases, the only and best solution is to send it to repair as this could be a hardware issue.
Be aware that during the repair process, devices get wiped, so backup your non-cloud data before.
As said before, i really recommend to send it to repair IF this issue started and wasn't there before.
Keep in mind that especially if you watch videos from a source outside the device there are mutliple things coming together and may run on high load which ends up in high temperature .
- Wifi or modem module to grab all the data from outside the device which depends on what kind of quality you watch (SD requires way less data than 4k just an extreme example).
- CPU/GPU/RAM to proceed the data
- Display driver to proceed the data to the device
And also the frame rate and display settings take effect. So it can be that this is expected behavior on your device as for example a high quality CPU will be less hot (as it may only need to run a part of it's full power) than a low quality CPU (as it may need to run at full scale to proceed the data).
So finally, there are many dependencies we can't figure out for you here. And i want to point to my first sentence before.
P.s.: I repaired several devices from different manufacturers and models in a certified service center so i have a bit experience in that. And i bet your desribed behavior is normal as a device defect normally shows up (regarding heat) when device is not used as well.
