If you get the following error:
Fatal error: During inheritance of Countable: Uncaught Exception: Deprecated Functionality: Return type of Composer\Repository\ArrayRepository::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in vendor/composer/composer/src/Composer/Repository/ArrayRepository.php on line 277 in vendor/magento/framework/App/ErrorHandler.php:62
Then you will need to make a couple of core edits in vendor to proceed after running composer install.
There needs to be two edits inside the /vendor/ folder to add #[\ReturnTypeWillChange] that fixes sites that are upgraded from 2.3.x and <=2.4.3.
The first file vendor/composer/composer/src/Composer/Repository/ArrayRepository.php on line 180.
The second is vendor/composer/composer/src/Composer/Repository/CompositeRepository.php on line 277.
Your files and line numbers may vary, so I recommend creating .patch files that can be run after each composer install.
After the return type will change directive is set, everything can proceed as normal.