The start of all processes is the init process which currently is systemd. This is the process that always has the PID 1 which identifies it as the top level process.
From the init process, everything else is started: as a child of init process. Children can have children for themselves as well.
In a parent-child relationship, the children depend on the availability of the parent. And if the parent goes away, then your child will be in problems.
If you will kill the parent, the children will disappear.
A thread is like a process within a process, running several tasks at the same time. That's what we call a multi-threaded process.
A job is a process that is running from your current shell environment. Ordinary users can manage their own jobs.