I had cause to want to clear down my mac mini and install Big Sur afresh. I erased/reorganised the drives. Formatted for APFS, and ran installer.
I now have a clean looking mac, but I never told entered the wifi credentials and yet it's rejoined the network. I have not entered any Apple ID/iCloud account details.
How has it snuck the wifi details through to a clean system?
Mac mini, macOS 11.5
That is why Apple Suggests - Reset NVRAM before for Selling, Trading in or giving away an Apple Computer . The same would apply to a Wipe and Reinstallation. If not performed, it may Auto-Join a Known Network and might happen on first reboot after the clean installation.
Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. Release the keys after about 20 seconds. This clears user settings from memory and restores certain security features that might have been altered.
That is why Apple Suggests - Reset NVRAM before for Selling, Trading in or giving away an Apple Computer . The same would apply to a Wipe and Reinstallation. If not performed, it may Auto-Join a Known Network and might happen on first reboot after the clean installation.
Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. Release the keys after about 20 seconds. This clears user settings from memory and restores certain security features that might have been altered.
Just to be sure that you did a "clean" install, did you follow the steps in the following two support articles?
... and NOT restore from a backup ... at least, not yet?
Big Sur install on to erased drives still remembers WiFi
