Once we are done with extraction, what are the ways to write netlist for the testbench? Other than manually writing it. Is there a tempelate we can use?

You can build yours using the existing python APIs. @Harsh Khandeparkar

Do you mean a prePEX netlist? You can build it using the Python API.

No I meant the testbench to setup simulation for my block. Testbench with input sources, supply and output load.

There is no template but you can use the opamp testbench as a reference