Search in google or look into the below websites to get more basic knowledge about investment.
Usually with a 401k the company would “match” your contribuitons to a certain limit. I haven’t heard of a 401k where they contribute even if you don’t. However, it could have happened. I would call the company that you are receiving letters from to see if you have an account. When you call, they will probably ask for your Social Security Number to confirm who you are.
Good luck