How do I redirect to different backend server based on URL using Apache?

You can use your Apache frontend Server as a reverse proxy for your backend servers : mod_proxy

<VirtualHost *:80>
   ServerName abc.mydomain.com
   ProxyPass / http://192.168.0.9
   ProxyPassReverse / http://192.168.0.9
</VirtualHost>

<VirtualHost *:80>
   ServerName def.mydomain.com
   ProxyPass / http://192.168.0.10
   ProxyPassReverse / http://192.168.0.10
</VirtualHost>

You will have to enable these modules :

a2enmod proxy 
a2enmod proxy_http

The advantage of mod_proxy is that it is transparent for the end-user : the URL in the address bar of their browser will stay unchanged : it will remain http://abc.mydomain.com but in fact they are served by http://192.168.0.100.

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