Please answer these kindly. I have been looking all over the internet but cannot get a clear answer.

I’m very new to Frappe and ERPNext. Here’s what I want to understand:

How exactly can I extend the ERPNext wep app? How can I write different modules similar to CRM, HR and Integrate them within ERPNext.

What is the point of running multiple apps in the same site? How do we decide which apps loads when entering the site? How can I make the apps communicate? Is building a separate app and adding modules the same as adding modules in the ERPNext app?

If I want to load my custom app throuh ERPNext just like CRM, Buying, Selling etc, how do I do that?
How do I list my app as a module on the side bar?

The new update doesnt let me add a module to a new workspace.

I tried creating a new custom app and installing it at the same site as ERPNext. But I dont know how to access my custom app features from here.

After Installing the custom app to the site, I can access them from Module List. But how exactly do I use the custom app features?