Worst case you boot up a virtual server with the same OS as your own and just go down the tree learning permissions, and it’s a deep dive learning experience.
The trick is that you loose access to every file on the system. chmod is also a file. And ls. And sudo. You see where it's going. System will kinda work after this command, but rebooting (which by a coincidence is a common action for "fixing" things) will reveal that system is dead.
Yep. You could run chmod again to fix it (from a different OS / rescue USB), but that would leave all the permissions in a messy state - having everything set to 777 is incredibly insecure, and will also likely break many apps/scripts that expect more restrictive permissions. So the only way to fix this properly would be to reinstall your OS/restore from backups.