what happens when shell script is built in bitbake

I am just tying to understand the bitbake build system. I have a doubt regarding how shell functions/task gets executed. I am going through below documentation https://docs.yoctoproject.org/bitbake/ In a part of documentation it says shell scripts are executed by /bin/sh. In another part of documentation it says "BitBake writes a shell script to ${T}/run.do_taskname.pid and then executes the script". What is run.do_taskname.pid? what exactly does it do? What exactly happens when bitbake encounters a shell script?