Learn English – A pair of scissors are or is

In this case you are talking about the option you choose in a game. You could read the sentence as

The option scissors cuts the option paper

or, abbreviated,

Scissors cuts paper

So while "scissors" is plural and you would always say "scissors cut paper", in this case the word refers to an option within the game (singular) rather then the actual item.

Compare with "scissors are better than ..... " if you are a fan of sharp metal things, but if you are talking about the game you'd get "(the choice of) scissors IS better than ..."

Jeans is a plural noun like pants/trousers.

So you cannot say I wear a jeans. Instead, you usually say I wear jeans, though you can also say "I wear a pair of jeans".