a little correction,
how security screening works, It depends on the IRCC officer if he wants to send you for a security screening due to your application hitting one of the indicators in their system
so they sent you for a security check, first, your files go to CBSA and then CBSA sends it to CSIS, once CSIS confirms your security is concluded and the file has been returned to CBSA, now your file is with CBSA, your gcms notes will start showing security in progress by CBSA, and once CBSA did their part they will send your file back to IRCC and then IRCC officer will make a decision
so in easy words IRCC=>CBSA=>CSIS
IRCC<=CBSA<=CSIS
hope this helps