Extent of sampling
The analysis here of the general election 2019 is based on a nationwide post-poll survey (The National Election Study 2019) conducted by the Lokniti programme of the Centre for the Study of Developing Societies (CSDS), Delhi.

The fieldwork for the post-poll survey was conducted in 26 States after each phase of voting which began from April 12, 2019 and went on till May 21. A total of 24,236 voters were interviewed in 211 parliamentary constituencies (PC) in 26 States.

Nature of survey
The National Election Study 2019 (NES 2019) is a post- poll survey conducted during the general election of 2019 by a team of scholars from all over India and coordinated by Lokniti, Programme for Comparative Democracy at the Centre for the Study of Developing Societies (CSDS). The NES 2019 is a large and comprehensive social scientific study of India’s national elections and continues the series begun in 1967 by the CSDS (with a break between 1971 and 1996).

Difference between post poll survey and exit poll
It must be noted that the post poll survey conducted by the CSDS is very different from an exit poll in which voters are approached outside the polling booth on voting day. Instead, voters who were randomly selected from electoral rolls were approached by the field investigators for an interview at their place of residence after votes had been cast in their respective constituencies but before the results were known. The purpose of the post-poll survey was not just to try to understand voting behaviour but, more importantly, to understand the reasons why voters chose the parties and candidates they did.

(Source:https://www.thehindu.com/elections/lok-sabha-2019/the-method-behind-the-sampling/article27249435.ece)