The general concept is "job control". You can start with https://en.wikipedia.org/wiki/Job_control_(Unix) for more reading.
pkill -9 bash
when I both sleep 200 &
and sleep 202
running, both processes get re-parented to PID 1. This is the expected behaviour for that case. A normal kill doesn't work. The shell appears to ignore the signal since it is still running something.