How do I transfer data from my old iPhone 6 to already set up iPhone 11

you would want to restore the iPhone 11 from a backup of the iPhone 6.

You cannot transfer everything and leave the iPhone 11 'still set up'.

If the data you want to transfer is just iMessages and/or Photos that you store in iCloud, then you would just turn on the switch for Messages/Photos in Settings > [your name] > iCloud. (must be using the same Apple ID).

Have a few questions:

can iTunes on 2011 iMac w latest OS be used to backup a new iPhone 12?

not all data from the iPhone 6 was transferred to the 12 at start of use of the the 12 iphone. Is there any way of picking and choosing and copying data from the 6 to the 12 iPhones? If not how can I populate the 12 with data like keychain, mail configs, calendar, etc. data?

in the long run, are the iMac and iPhone 12 devices so different I cannot backup the iPhone 12 on the iMac?

last: the recording indicated to get the iPhone 6 data on the 12, I needed to use the reset function on the iPhone 6. If so, can a better set of instructions be written to aid in moving the data from the old to new iPhone.