I took apart a broken calculator to show my daughter how it functions. It had a small "traditional" FR-4 PCB mounted underneath the LCD glass, but the pushbutton area was printed on a paper-like substrate:
The paper is plasticized and shiny, but there are fibers present if it is torn. It has the equivalent of plated-through-holes. The conductors are thin, matte-back material that look similar to toner.
Where the paper meets the FR-4, the traces connect together with some sort of adhesive (thin glue or tape). In other words, the copper traces are covered by matching "toner" traces, and affixed somehow...
The calculator was purchased at a Dollar Store for, you guessed it, one dollar :)