I am not sure if you are asking the right question but I will try to answer.
1) Every letter is not patented. I can draw a triangle and so can other people, nobody can patent the ability to draw a triangle.
Similarly, nobody can patent something like all games where you shoot from bottom belong to me - there is no such thing.
Patents must be very precise.
Consequently, you can do a lot of things which other games do. And as you might know it is very spread in the gaming world copying best practices from other games. The only thing you cannot or should not do is copy 100% exactly.
2) Much more often the graphics, brands, names and design is patented and not the code or game logic behind. Like you cannot make Monopoly because they might have a problem with you, but you can make your own version which is slightly different and call it COW-WORLD.
Finally, why I said you are asking the wrong question is: what you are asking is referred to cloning i believe. There is indeed a purpose or place for game clones. But long term wise, if you only clone stuff and never create something that is truly your creation / something unique, then your success may be very limited.