can we restore a back up from Iphone 8 device to iphone 6 ?

You will have to import your photos and videos to a computer (along with any other data files) and sync (or copy) them to the "new" iphone 6

Apps like Whatsapp may allow you to backup to icloud and restore. Text and other social network history may be lost.

Only if your iPhone 8 is not upgraded to iOS 13 or newer.

You can't restore a backup from a newer version of iOS to an older version of iOS.