I think this screenshot shows enough. Well, the problem there is it doesn’t actually load an HTML file (the owncloud dashboard) but it shows the content of the PHP file.
Seems to be desame as https://askubuntu.com/questions/451708/php-script-not-executing-on-apache-server . But I have it on a raspberry pi which doesnt fix with that.
This is also not an ownCloud problem.
As you have already correctly found out mod_php is not enabled on your RPi.
Now you just need to enable the correct one, that is installed on your system.
Probably something like
libapache2-mod-php7.3, you could run
dpkg -l | grep php | grep mod.
apachectl -M you can figure out the modules loaded into your apache.
Please paste the output of both commands.