Britain had an entire program during WWII designed to identify, capture and turn German agents to get them to feed disinformation to their superiors:

https://en.wikipedia.org/wiki/Double-Cross_System

It was used in a clever manner to subtly suggest to the Germans an incorrect location (Pas-de-Calais) for D-Day.

WWII produced some of the most interesting (now publicly known) espionage operations. My favourite is Operation Mincemeat:

https://www.damninteresting.com/mincemeat-and-the-imaginary-...

Another way the Allies misdirected the D-Day landing sites was to do a lot of reconnaissance flights to the southern coast of Bretagne (northern bit of the west coast of France). Of course, a lot these recce flights also 'just happened' to also overfly the actual location of the D-Day landings...