Here is a report from the US that says the SAA has not yet...

Here is a report from the US that says the SAA has not yet deployed in Manbij.  Presumably the SAA is moving to areas close to  Manbij.

https://au.news.yahoo.com/syrian-army-not-manbij-city-us-military-180632119--spt.html

This is not surprising given that Manbij is currently defended by the Sunni Arab controlled Manbij Military Council which is aligned to the SDF.

The Kurdish YPG essentially left Manbij some time ago at the behest of the US.

In other words the Turks are facing off a Sunni Arab force on  the Manbij front (for now).  This fight has everything to do Turkish  territorial expansion.  It has nothing to do with Turkey's civil war involving the PKK.

The main  Turkish attack   will be as per the map Asteroider posted above in this thread. The  attack will come from the North and  overwhelming firepower will be applied.