How can I save game “professionally”?

I am a novice programmer and just learned about the concept of saving and loading games. The way I am doing it is to write data into a text file and read it afterward. However, I realize how insecure this method is. What is going to stop someone to just open the text file and edit it? Furthermore, after going through game folders of popular games, I do not seem to find any text file dedicated to saving data at all. So there must be more advanced saving methods than simply writing and reading from a text file. What are they then and how can I do something similar to that? (I mainly use Java and C++)