There is a Related Items sublist under the store subtab on the inventory item record, is there any way to move it to main(the main section)?

Customization > forms > inventory form > move elements between sub tabs > lists.

There is no option there to move it to main

You’re correct. Is the primary tab not sufficient?

No the client wanted it on the main page, to see the info without opening tabs.

In my suggestion it would be available without opening any tabs, but you’d have to scroll down to see it.