The members of the Constituent Assembly which drafted the Constitution of India were

The Constituent Assembly members were chosen by indirect election by the Provincial Legislative Assemblies created under the Government of India Act, 1935 — i.e., members of provincial assemblies elected the representatives to the Constituent Assembly — so they were elected by the Legislative Assemblies of various provinces [2]. The elections for the 296 seats allotted to British Indian provinces were held in July–August 1946 by proportional representation (single transferable vote); the Indian National Congress won 208 seats and the Muslim League 73, confirming the provincial-assembly basis of selection [1]. (Representatives of princely states were to be nominated by their rulers, making the Assembly partly elected and partly nominated.)

Sources

1. [2] https://static.pib.gov.in/WriteReadData/specificdocs/documents/2021/nov/doc2021112621.pdf
2. [1] Laxmikanth, M. Indian Polity. 7th ed., McGraw Hill. > Chapter 2: Making of the Constitution > The features of the scheme were: > p. 12