Is it possible to take an unknown Arduino UNO, plug in the USB cable, and view what ever sketch is on it, on the computer screen?
Ken
A search on Google returned 336,000 results.
'extract code from Arduino'
No. The best you can do is download it's hex file, it the lock bits are not set, using avrdude commands, and run decompiler on in. Then try and make sense of what you see.
For example, put a known file into this
(compile a program with File:Preferences and Verbose output turned on so you can see the path where the .hex file ends up)
and see if you recognize what you get back:
https://www.onlinedisassembler.com/odaweb/
Under Platform, select one of the AVR choices. I don't know which one relates to the Atmega 328/2560/1284.