My company needs a huge application with a combination of subapplications. So I would like your opinions for the structure of the application. Let's say that I have a main application (Extranet) with sections Inventory, Managing, Trading. It's section is a separately application. For example Inventory has forms for hotel details, type of rooms, room prices etc, Managing has another 4-5 forms etc. The same with Trading. My question is how can I connect these 3 subapplications in the main Extranet application? Because I would like to control these 3 subapplications from the Extranet application.

The example which I have in my mind is the zoho mail where it has different sections like Docs, CRM etc. and every section directs in different application.