How do I know which Bluetooth chipset is being used by the iphone?
I would like to know if the Bluetooth chip has changed between iPhone 11 series and iPhone 12 series

The chip is likely to be slightly different than last year's model due to the natural progression of technology, but the vendors are likely to be the same.  Since 5G is being rolled out, that increases the odds of the chip itself being different.
Broadcom strikes $15 billion deal with Apple to sell it iPhone parts

I would like to get mode details including the chipset manufacturer, model number etc.

A tear down is always a good source for this type of info

Apple iPhone 11 Pro Max Teardown
iPhone 12 teardown reveals simpler internal design, 5G radio details

Keep in mind it has been Apple's prerogative and practice to obfuscate this information so while you might get model numbers, you may not be able to access data sheets on these chips due to contractual agreements.