How to use C++ Session Search Results with Blueprints?

They actually have a wrapper struct so they can use it in blueprint

 USTRUCT(BlueprintType)
 struct FBlueprintSessionResult
 {
     GENERATED_USTRUCT_BODY()
 
     FOnlineSessionSearchResult OnlineResult;
 };

Note that this struct is only usable in blueprint and not in your code since the struct is defined in the engine files.

So if you wanna have c++ blueprint callable functions that implicitly handle FOnlineSessionSearchResult on blueprint you will have to create your own wrapper struct for it, so your functions can handle them in c++ but they are exposed in blueprint.