Is a BLTouch probe used during a print after the first layer?
Your assessment is correct, after probing before printing it deploys several times, after that it is stowed till the end of the print unless you call for deployment.



The probe is used by G-code command G29; this command is used by a few printer firmwares to perform a bed probing sequence where, depending on the options set in the firmware, the surface of the bed is scanned by deploying the sensor at various locations. From these measurements, the firmware will determine the shape or tilt of the bed to compensate for deviations in Z height during the first couple of layers or millimeters of the print. Basically, it will keep your nozzle (approximately, as it is based on calculations) at the same distance all over the bed area.

Once the print starts after the G29 command, the sensor is stowed until the next print calls the G29 command (or M280 with arguments to deploy on Marlin/Duet/Smoothieware, or M340 on Repetier firmware).