process ps session – What are ‘Session Leaders’ in `ps`?

Process groups exist primarily to determine which processes started from a terminal can access that terminal. Only processes in the foreground process group may read or write to their controlling terminal; background processes are stopped by a SIGTTIN or SIGTTOU signal.

You can send a signal atomically to all the processes in a process group, by passing a negative PID argument to kill. This also happens when a signal is generated by the terminal driver in response to a special character (e.g. SIGINT for Ctrl+C).

Sessions track which process groups are attached to a terminal. Only processes running in the same session as the controlling process are foreground or background processes.

It is not possible to determine process groups or sessions from the PPID. You would have no way to know whether the parent of a process is in the same process group or a different one, and likewise for sessions.

POSIX defines sessions thus:

A collection of process groups established for job control purposes. Each process group is a member of a session. A process is considered to be a member of the session of which its process group is a member. A newly created process joins the session of its creator. A process can alter its session membership; see setsid(). There can be multiple process groups in the same session.

All process groups belong to a session. The concepts aren't dependent though, so I wouldn't say that a process group is a concept which exists only within a session.

Background processes are given their own process group when they're created, so disowning them doesn't change their process group; disown only manipulates Bash's job table:

disown [-ar] [-h] [jobspec ...]

Without options, remove each jobspec from the table of active jobs. If jobspec is not present, and neither the -a nor the -r option is supplied, the current job is used. If the -h option is given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. The return value is 0 unless a jobspec does not specify a valid job.