It depends upon company to company like many companies just verify your information with your previous employer but then there are companies who go beyond your previous employer and can contact your other employer’s if you have any, see these things are pretty serious especially when the job involve a technical expertise.

In my company many employees had given fake experience certificate and also they forged several document and when the company send them to be genuine they lied. The company was more active and know how to expose their lies so they hired a professional company by the name of Candidate Identity Verification – CelebratingLife who by using AI track down all their information and exposed them completely and than company fired those employees and also they filed an FIR against him for providing fake information so it depends how much company is serious about the background check.