I filed a petition for my parents to immigrate to the US and it was approved a few months ago. We have not heard anything since then. What usually happens after the petition approval

After the petition is approved, the case is generally sent to the National Visa Center for further processing if your parents are outside the United States. The center collects fees, civil documents, and financial sponsorship forms before scheduling an interview at a US consulate. The timing can vary, but once all required documents are submitted and accepted, the case moves forward to the interview stage.