Install extensions for PHP 7.4 on Ubuntu 20

sudo apt-get install php7.4-cli php7.4-fpm php7.4-bcmath php7.4-curl php7.4-gd php7.4-intl php7.4-json php7.4-mbstring php7.4-mysql php7.4-opcache php7.4-sqlite3 php7.4-xml php7.4-zip

or 

sudo apt update

sudo apt install php7.4

sudo apt install php7.4-common php7.4-mysql php7.4-xml php7.4-xmlrpc php7.4-curl php7.4-gd php7.4-imagick php7.4-cli php7.4-dev php7.4-imap php7.4-mbstring php7.4-opcache php7.4-soap php7.4-zip php7.4-intl -y