How to get sub page like this?

What are you trying to accomplish with this?

What do you consider a sub page to be? Content farther down on mainpage or content on a separate page that is subordinate to mainpage? While ?download=links could achieve this, it’ll take some custom coding. There are alternatives that don’t require custom coding. To jump to a section on the same page, use an URL like:
example.com/mainpage/#links

The links section will need a HTML tag whose id attribute is “links”.

To link to a subordinate page, when you create the page, specify mainpage as its parent. Then its URL would be something like:
example.com/mainpage/links/

@sterndata I want to create a page without actually creating new page and want it be something like this example.com/mypost/subpage but how can I do that is there any plugin for that.

You want the content generated dynamically based on the parameter? That might take some coding. What would the content be?

Yes, you could do that, but I don’t know if there’s a specific plugin for modAPKs. It could be done with, for example, WooCommerce or Easy Digitial Downloads. Will you get those specific URLs? No, but you’ll be able to run a download store.