sudo apt install php-xml will work but the thing is it will download the plugin for the latest PHP version.

If your PHP version is not the latest, then you can add version in it:

# PHP 7.1
sudo apt install php7.1-xml

# PHP 7.2:
sudo apt install php7.2-xml

# PHP 7.3
sudo apt install php7.3-xml


# PHP 7.4
sudo apt install php7.4-xml

# PHP 8
sudo apt install php-xml

Ref: https://stackoverflow.com/questions/43408604/php7-install-ext-dom-issue