In layout view page, when admin select design from dropdown it will redirect to layout/designer/id. Can you please tell from where layout designer route is get called ?
Thank you for your message. My apologies but I’m not sure I understand your question. Could you clarify what it is that you would like to know and what it is you are looking to achieve?
Many Thanks.
Firstly, Thank you for the soon reply 
When admin click on Design button from dropdown of layout view page(table view). I searched for dropdown code but i didnt get it. From where the route is calling when admin click on Design button(Selected in image which is in blue border color)
Thank you for your reply. The Design button you are referring to quite literally does as you mentioned in your original message, it will open the link <CMS_URL>/layout/designer/<ID number of Layout>.
Many Thanks.
I know that open the link which I mentioned in first issue. I am asking from where the route is calling <CMS_URL>/layout/designer/<ID number of Layout> . Whether it is calling from twig file or somewhere like how the sidebar route calls.