> You took an extreme example

True! It was a rhetorical point, but similar examples abound in the Tier 3/4 cities and small towns that represent the majority of India.

You're still at the mercy of the DC's office and the associated State PSC to let the transaction go through, and local bias will abound. And in these kinds of places, if you get into a land dispute, the entire apparatus will rally behind the local even if they are in the wrong, because the local can leverage their local family/social network.

> Same Constitution, same legal framework

Absolutely, yet dependent on state PSC to implement. And local customary laws can often take precedence over central rules and regulations due to Article 13(1).

> There are many all-India services and people are transferred all across India. Many work in different states than those of their home state

There are, yet at the end of the day, Home Bias remains, as IAS officers posted outside their home state are significantly less likely to climb up the ladder and tend to get hamstrung [0].

Anecdotally, in the early 2000s, my ancestral district got an ethnic Tamil DC/ADC, but they were completely frozen out by the local panchayat, MLAs, and MP because they were viewed as an "Outsider", and the man was quietly transferred within 2 years and an ethnic Punjabi officer was brought it (still an "outsider" but viewed as "closer").

> Same religion

At a broad level Hinduism sounds unifying, but in action, the regional variations are massive.

It doesn't matter as much to sharyi/city folk, but local deities and practices vary massively and what one regions treats as "Hindu" can appear entirely alien to another region.

Tamil society doesn't bat an eye at cousin marriage while that would be grounds for a honor killing in HP/PB/HR. Meanwhile, in my region we revere a number of Muslim mystics like Lakhdata and in some cases even practice Muharram (Hussaini Brahmin), but to a Hindu from Gujarat or Karnataka, that would appear Muslim.

> I think if you go deeper you will notice the unifying characteristics

There absolutely are unifying characteristics, but I think these are much more prominent in Tier 1/1.5/2 cities which are melting pots.

Most Indian urbanization is being driven by Tier 3/4 cities which tend to be much more insular.

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Big picture, I think differences are significant when outside the Tier 1/2 cities, but this is part of the power of Indian federalism.

The loosely coupled nature of Indian federalism allows regional ethnic identity to continue to exist with a unified "Indian" identity and act as an outlet to ethnic insurgency.

This is how ethnic insurgents in NE India were able to merge into the BJP in the 2010s, and regionalist and linguistic parties such as Shiv Sena, DMK, TDP, TMC, etc are able to create loose political alliances and coalitions with "national parties".

Also, this imo is a major reason why BJP has been so dominant over the past decade - they are able to co-opt localist movements into the state branch of their party.

The INC used to be able to do this, but these local leaders split off to create their own parties by the 1990s.

[0] - https://www.nber.org/papers/w25389