by default the left Layered navigations are hidden in Luma template. How do I un-hide the it. similar to blank theme.
The file which generates these for the Luma is in \vendor\magento\theme-frontend-luma\Magento_LayeredNavigation\templates\layer\view.phtml
On line 36 you'd change
"active": false to
"active": [0,1] if you want the first two categories to be expanded,
"active": [0,1,2] if you want the first three expanded etc.
Note that you should never change core code, and so you'd ideally need to override this file in your own extension theme. (However, I am aware that Luma doesn't seem to be great for extension themes.)