If you check enabled php modules with php -m and there is not any memcached entry in the list, then you have to check the memcached extension configuration file.
vim /opt/homebrew/etc/php/7.4/conf.d/ext-memcached.ini
In this file above if the extension given by a path then you have to modify the line without any path and extension name only. (Note: For any after exception probability you may choose to comment the line with the path information.)
#extension=/opt/homebrew/opt/php@7.4/bin/memcached/modules/memcached.so
extension=memcached.so
After that modification restart the php 7.4 services with
brew services restart php@7.4
Without any further error or warning the php@7.4 service should restart after this command above.
Lastly check the module with php -m and memcached should be over there.
Finished.