To add to @dialabrain points
There are several points to consider before attempting the upgrade to Big Sur. The most important is Due Diligence on the part of the User and is a Prerequisite for a smooth upgrade.
The below points should all be consider Before attempting the upgrade.
Point # 1 - Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - many will / may carryover to Big Sur making the system unstable / unusable.
Point # 2 - What are the System Requirements ie: Empty Space Requirements specific to the Big Sur.
Point # 3 - Are the installed Applications / Extensions compatible with Big Sur. Check with the Developer for compatible Apps / extension or do they offer updates. Same does for Third Party Drivers for Printer, Scanners etc.
Point # 4 - Do I have a Rescue Plan In-Place. This comprises having a Time Machine Backup and for addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways
Point # 5 - 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.