Interesting information by Joseph P Chacko at frontierIndia. com titled “Google Maps Show Indian [Arihant class] Submarines Berthed Alongside” of February 5, 2021:
...Initially [what became INS Arihant] was conceived as SSN based on the [Soviet] Charlie1 class design
...However, after the first concept design was made the requirement to make an SSBN post Pokhran 2 (aka II) in May 98, a decision was made to make Arihant as an SSBN.
The objective was achieved with the design collaboration with Rubin design bureau in St Petersburg.” Much more.