The fan is loud on my MacBook Air

It indicates some Run Away Process consuming a lot of CPU and / or Memory. Open Activity Monitor >> View >> View All Processes and check CPU Tab and look for process using unusual amounts of CPU and Memory Tab and look for same unusual Memory usage. You might be able to identify the App or Process.

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. The Built-in Security of Big Sur is all that is required.

Try resetting the SMC How to reset the SMC of your Mac - Apple Support