What is difference between autonomous campuses...and affiliated campus interms of placements... Would affiliated campus get less placements?
IITs are what you call autonomous Institutes.
They can award degrees in their own right. They do not require UGC/AICTCE approval of their course structures.
Other than IITs, IISERs, IISc, RGIPT, NITs, some IIITs, AIIMS etc… are the family of institutes that are called Institutes of National Importance. These are autonomous Institutes and are known for their academic excellence. (Of course some NITs /IISERs aren't exactly great at placements and academics).
But although there's no data available that directly compares college in this manner, but autonomous institutions might have an edge.
Although comparing any two colleges, in my opinion these factors matter less than - Academic repute, Infrastructure (very important), Research output, Placements, exposure etc…