How to get all the record names in Item Group based on having parent item group field exits
I’m not sure in what context you’re trying to get all items by group, if you mean in Item List view, currently is not implemented. It was started in fix: Descendants of including self by netchampfaris · Pull Request #15360 · frappe/frappe · GitHub but wasn’t finished.
If you’re developing some custom feature, you can use this PR as a guide how to get child item groups.