Should I reset my MacBook Pro after a major macOS update? I've already installed two major updates (Catalina and Big Sur) on my MacBook Pro 2019. Is it advisable to reset (format) my machine for it to work at its best, or would it not make much of a difference?
Unless you are experiencing serious issues (such as malware or other unexpected behavior), there's no need to ever reset/format a Mac. Even if you are having problems, it doesn't mean you need to format it. If you know the cause or are willing to troubleshoot the issue, you could fix most problems without formatting your Mac.
Macs have separate volumes for the OS and for user data.

If you reinstall macOS, it only makes changes to the OS volume, and you will not even realize macOS was reinstalled because it will reboot and everything will remain exactly where it was, even any windows you had open before reinstalling macOS.
If you format the data volume, you will lose all of your data, but the OS will remain functional.
If you format the whole drive, you will lose both volumes and have to reinstall macOS, after which it will ask if you want to restore your data from a Time Machine backup.

Personally, I've been updating macOS on my devices for several years without formatting, I guess ever since macOS Mavericks. When upgrading to another Mac I just restore my Time Machine data or use the Migration Assistant (both are built-in on macOS).
It's always advisable to have backups of your data. On a Mac, you can setup Time Machine and keep it running. Your Mac will create hourly backups for as long as the Time Machine drive is reachable. You can then restore all your data in case you upgrade to a new Mac or something unexpected happens.
You can also use cloud services to keep some of your files safe. iCloud Drive is built-in on macOS.
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