First of all, clean up everything relevant including packages, as you seem to have no idea where you are. Then start with simple Apache. Is it working fine? Learn making small (tiny) modifications in the config and checking if they work. Once you're fine with the basics, reach for the PHP mod for the same Apache and see what can be done with that. Once you'...
Proper answer from comments:
The upgrade probably did not install the libapache2-mod-php7.3 package, which is required to integrate php7.3 into apache2.
To check if it is already installed, run
dpkg -l | grep ^i | grep libapache2-mod-php7.3
If this doesn't returned anything, it needs to be installed.
apt install libapache2-mod-php7.3
Afterwards, disable ...
You will need to have your web server (you are fetching the file from a web server, right?) to interpret php. Most likely, you don't have php installed.
For instance, if the web server used apache on a Debian-like system, you could need install the package libapache2-mod-php. If you were using nginx, you should configure it via fastcgi, etc.
The exact steps ...