No, the core of Hindi is inherited, ultimately from something like 'vulgar Sanskrit', not borrowed from Sanskrit. So like the core of Italian vocabulary is inherited from (vulgar) Latin, though there are of course also direct borrowings (at later dates) from Latin into Italian.

So 'fire' in Hindi(/Urdu) is āg (आग), but 'fire' in Sanskrit is agni (अग्नि), which has also been borrowed into Hindi (as the name of the god of fire; or fire in ritual contexts). Now, āg descends from agni, but represents the natural linguistic development into Hindi.

That's why I said Pali, or something like Pali. Pali words seem like they are vulgarised Sanskrit words.

Fire in Pali is 'aggi'. Seems a plausible link between 'agni' and 'aag'.

Well, Pali is a sort of standardised form of Middle Indo-Aryan, so, yes.

But you suggested there would be very little left. Yet the core of both Hindi & Urdu is (unsurprisingly) inherited from Vulgar Sanskrit>(some sort of) Prakrit>Apabhramśa>... , so it's rather a lot of vocabulary that's there.

You’re right, my sloppy remark was incorrect. Is there any reading you can recommend in this area, anything that may come to mind easily?

For the more modern vocabulary issues (borrowings from Persian, Perso-Arabic, English, Portuguese etc.) in Hindi/Urdu, I would recommend the introduction to

(1) Christopher Shackle & Rupert Snell. 1990. Hindi and Urdu since 1800: a common reader. New Delhi: Heritage Publishers.

[which is actually now freely available online at https://repositories.lib.utexas.edu/handle/2152/46072 (had I known, I would have linked it earlier/higher in the thread, since it's very relevant) its bibliography is also good for further reading]

For the earlier development of modern Indo-Aryan languages from Sanskrit:

(2) Jules Bloch (ed. & translated by A. Master). 1965. Indo-Aryan from the Vedas to modern times. Paris: Adrien-Maisonneuve.

For a good general overall (including history and other things) to Indo-Aryan languages:

(3) Colin P. Masica. 1991. The Indo-Aryan languages. Cambridge: CUP.

In Hindi (modelled on S.K. Chatterjee's The Origin & Development of the Bengali language [written in English!]):

(4) Tiwari, U.N. 1961. हिंदी भाषा का उद्गम और विकास [hindī bhāṣā kā udgam aur vikās]. Prayag, Allahabad: Bharati Bhandar.

Another great (free, online) reference is the Digital Dictionaries of South Asia (including non-Indo-Aryan languages of India as well) at: https://dsal.uchicago.edu/dictionaries/ (perhaps esp. including Turner's A comprehensive dictionary of Indo-Aryan languages: https://dsalsrv04.uchicago.edu/dictionaries/soas/ ).

[Some of the other references can be found 'freely' online in pdf form in the usual 'dark' corners.]

Wow, wow. This information is greatly appreciated, many thanks.