The latest custom chip from Apple is the security focused T2. It was first used in the late 2017 iMac Pro, and can now be found in the updated MacBook Air and Mac mini launched 30/10/2018. Special features of the chip include the ability to securely store biometric data like fingerprints, and the ability to encrypt and decrypt the built-in solid state drive, therefore making it ideal for Macs with a TouchBar.
Full details of these features and many more can be found in Apple’s T2 Security Chip Security Overview Document.