There might be an incompatible Application installed, Anti Virus software installed or just a runaway process using a lot of CPU and Memory. Open the Activity Monitor >> View >> View ALL processes. Check in the CPU Tab for unusually high CPU usage by process name and do the same for Memory Tab. This will help identify what is causing the Hot Air and Fans issue.
Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.
There are no known Viruses in the wild that self replicate and affect macOS. There are Malware and Adware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application.
Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.