Hm, that’s a wild mixture of messages…
Do you happen to have any whitespace of the top of your index.php or config.php?
Then again there are these messages that refer to seemingly missing PHP extensions. Try to disable them all in your php.ini, or try using a different PHP version.
See also (don’t know if you have also upgraded Mamp and your OS to Big Sur): Updated MAMP Pro to v6.0.1 nothing works - #17 by meltajon