Early games will establish things like "can ghosts go through each other", "if I hit the wall do I die" (like in Snake; though on Acorn Electron you could eat the wall sometimes!), "can ghosts go through the shortcut", "how long do pills last", do all pills last the same amount of time".

Then later efforts would be needed to know things like "what algorithm do the ghosts follow", "does the same colour ghost always follow the same algo", "do ghost algos change when you eat a fruit", "what order do the fruit go in" ...

Of course you could guess a lot of these, but that's not what "fully understand how the game works" means.