PIC logging question

Can you clarify for me the FAR definition of "pilot-in-command time" and
"flights as pilot in command" ?

Regarding the commercial pilot certificate with a glider rating,
61.129(f)(1) requires at least 100 flights in a glider as pilot in command.

Regarding logging pilot-in-command flight time, 61.51(e)(1)(i) says a
private may log PIC time only for flight time during which that person is
the sole manipulator of the controls.

Obviously, flying solo or taking a passenger is PIC time. What about a PPG
flying with an instructor? I'm told that instruction time after receiving
one's private cert can be logged as PIC, assuming the private and not the
instructor is doing all the flying. Is that your understanding?

Then 61.51(e)(4) says a student pilot may log PIC time when flying solo. Is
that your understanding? I did not log it that way, since there's a separate
column in my logbook for "Solo"

If those are both true, then I have a lot more PIC flights toward my
commercial than I thought !

Thanks.