The only way to search for items stored locally on your Galaxy is by going to the App screen (swipe up on the home screen).
To opt out of Samsung's promotions, head over to Settings > Samsung account > Security and privacy and turn off Get news and ...
You can clear the entire cache on your Samsung Galaxy phone, or clear only the cache for a specific app. Here's a guide on how to do both.
You can clear your Twitter cache in the app on your iOS or Android device to clear space and help it run better.