i am having trouble downloading big sur on my macbook

Make sure you have at least 50-60GB free space. Backup and uninstall any "cleaners, optimizers, virus apps, VPN etc". Connect the Mac via Ethernet to the router, restart the router and keep other network traffic in the household to the minimum. Boot the Mac into safe-mode and then try to update Big Sur or download the full Big Sur installer and apply it.

Agree with all @Matti Haveri has outlined.

Especially if there are AntiVirus software installed. Even if it is Deactivated - the softwares will interfere with the whole process. Besides - There are no known Viruses in the wild that self replicate and affect macOS and best to Uninstall as per Developers Specific Instructions.