The data for those maps in NYT are generated by the Carter Center from non-government contacts on the ground. In order to generate maps with that level of detail, they would have to have 10,000 deeply embedded individuals within all parties involved in the conflict. Or they could be simply generating a fantasy world.
No matter how crazy the scenarios become, we must remember that God is still in control. And Vladimir Putin.
They are (mostly) posters on social media...
That's in addition to more conventional sources.
There's an incredible amount of "data" out there. That said, most of it is surely of dubious reliability (think: 'Baghdad Bob'). Certainly much of the fine detail in the map is iffy. A question for me would be how many people with expertise in evaluating such information would the Carter Center have? It'd have to be a substantial group.
Note that the Times makes a really dumb error: various rebel forces are stronger in the north and northeast.
The Northeast? NYT can't even read a map?
Nonetheless, the general situation as depicted is probably not too far off, from all I've read, the past year or so. It actually makes sense that Putin is hitting the non-ISIS rebels - Putin's short term goal is to prop up Assad, not worry about who he needs to hit to do it.