Background check is mostly across the security and criminal records. RCMP and CSIS are the designated bodies to carry out the criminality and security checks respectively. It's not performed for all TRV applicants but VO makes the case by case decision. There are global databases which are used by RCMP and CSIS to check the applicants' background.
If you have lived in more than one country (for more than 6 months), ideally you should submit PCC from each one of the country you have lived since you have turned 18. That helps in expediting the background check for your case.