Are file permissions on macOS cryptographically enforced? Are permissions set in the 'Get info' screen when right clicking on a file enforced cryptographically? If not, how are they enforced? Could anyone, for example, boot of a live OS on a USB stick and edit file permissions?
The file permissions are only enforced by the operating system. If you boot from another OS (where you have root user access), you can read anything. 

Of course, if the disk is encrypted (research Full Disk Encryption), then you would need the key to decrypt it before you could mount it. On a Mac, the disk encryption key is typically protected by the users password and decryption is handled during login.