Are you dealing with sid? You are a brave warrior.
what does your php -v
say ? Alternative you can use to check it out:
apt-cache policy php7.4
Does it really say you have 7.4 installed?
Okay if it says 7.4 installed, then install php7.4-mysql
apt install php7.4-mysql
(note that it depends on libc6 >= 2.15)
if you do not have apt then you can download php7.4-mysql from here:
https://packages.debian.org/sid/php7.4-mysql
anyway you'll need all the following packages in the end : php7.4, php7.4-cli, php7.4-curl, php7.4-gd, php7.4-intl, php7.4-json, php7.4-mbstring, php7.4-mysql, php7.4-opcache, php7.4-readline, php7.4-xml, php7.4-xsl, php7.4-zip, php7.4-bz2, libapache2-mod-php7.4
and then after install
cat /etc/php/7.4/apache2/php.ini | grep extension=mysqli
;extension=mysqli // uncomment it
cat /etc/php/7.4/cgi/php.ini | grep extension=mysqli
;extension=mysqli // uncomment it
cat /etc/php/7.4/cli/php.in | grep extension=mysqli
;extension=mysqli // uncomment it
in the end reboot your box.
wish you good luck.