Could be any or all of the above, but as a former journo, I know that the 'good' journalists/columnists/commentators (i.e 'trustworthy' to the leaker, or well-embedded) get 'off-the-record' briefings from all kinds of people.
That could be a phone call (but those can often be traced) or a suggestion to go to lunch together where many things can be discussed, or a walk outside in a park, or even a simple email from an 'anonymous' email address.
Senior people do it for a number of reasons:
1. to further their own agenda within the organisation they represent
2. To frustrate the progress of a rival individual
3. To prepare the ground for some kind of future move
4. To build trust with the journo, so that they can call in a favour later..
Could be many things.
The 'off-the-record' means that the journo is not supposed to quote the source, but often, the journo and the leaker agree a 'form of words' that is acceptable to both sides. That is what subsequently appears in the media outlet concerned.
In reality, with modern phones, recording devises and so on, ther eis no such thing as an off-the-record briefing.
But if as the journo, you screw up and reveal the source, it will be a long time before you get any more such scoops.
IN theory the senior leakers keep the press office informed of their actions. But in practice, much of this kind of thing goes on under the radar.
IN the political journalism, there is the 'lobby system' under which certain journalists are given access to lobby briefings where senior party officials give the 'official' line on some incident, or plan, so that they get their message out to the newspapers and TV channels without being directly quoted.
Some newspapers have chosen to reveal the sources of those briefings and subsequently been barred from the lobby briefings.