Yes they contact authorities from the PA's home country and every other country where the PA spent over 6 months, the authorities check the PA's record to see if he/she ever had any record of convictions, arrests, offences including driving related offences, terrorism acts, fraud, if he/she ever been to jail/prison...
They also confirm the authenticity of all the documents the PA and sponsor sent them to make sure people are not commiting any fraud to get the PR in Canada
In addition, CBSA and CSIS check with other countries to see if the PA ever been refused a visa including visitor, immigrant, refugee or was deported for any act
So, CBSA and CSIS get in touch with all related authorities from different countries to do the background check, and when CBSA and CSIS hear back from them that everything is good/bad they report back to IRCC, then IRCC makes a final decision or sends a procedural fairness letter explaining what they found on the PA record and how it makes him/her inadmissible to Canada