If your Mac has a Broadcom BCM4331 chip (2011–2012), Wi-Fi will be broken until you apply the post-install root patches. You must plug into Ethernet during initial setup.
Non-Metal Macs (Intel HD 3000/4000) will have severe visual artifacts: Macos Big Sur Patcher
Even with a perfect patcher, things go wrong. Here is the troubleshooting guide. If your Mac has a Broadcom BCM4331 chip
Disclaimer: Back up your entire Mac using Time Machine or Carbon Copy Cloner. This process can fail, resulting in data loss. You assume all risk. Macos Big Sur Patcher