The full installer app is 12.26GBs in size so takes a long time to download.
Does your mac meet the requirements for Big Sur.
These are the macs that can upgrade to macOS Big Sur,
MacBook introduced in 2015 or later
MacBook Air introduced in 2013 or later
MacBook Pro introduced in late 2013 or later
Mac mini introduced in 2014 or later
iMac introduced in 2014 or later
iMac Pro
Mac Pro introduced in 2013 or later
You need to have at least 35.5GBs of free space before Big Sur will install.
When downloading have your mac connected to your router via cable.
Do not use the mac while downloading or any other devices that share the same
internet connection.
How to upgrade to macOS Big Sur - Apple Support