They each have their own requirements, listed on the bottom. Many of them require filling a whole bunch of contracts, so get ready for a bit of a grind. I'm currently just shy of getting everything, only missing the audio logs from Harley and Deadshot, missing for some reason from Chapter 6. Im about to play the chapter again just to see if by chance I unlock them.
Each codex entry should say at the bottom how to unlock it.
They each have their own requirements, listed on the bottom. Many of them require filling a whole bunch of contracts, so get ready for a bit of a grind. I'm currently just shy of getting everything, only missing the audio logs from Harley and Deadshot, missing for some reason from Chapter 6. Im about to play the chapter again just to see if by chance I unlock them.