What meta-data does iCloud drive save and restore for each file or directory? Observation: Machine A had HPFS+ with iCloud drive subscription. Machine B had APFS and joined the same iCloud drive. When the content was downloaded, all the directories had a fresh modification date on them.

Experiment / Observation 2: Machines A and B are using the same iCloud drive. Disconnect the network connection on B. Create and empty directory on A. After some time connect B back to the network. The empty directory gets downloaded on B but with a fresh modification time (time of download on B). This does not happen for files.

Setup: In the above "experiments / observations", machine A is running macOS 10.12.6 on an HPFS+ (case-sensitive and encrypted). Machine B is running macOS 10.13 beta on an APFS (case-sensitive and encrypted).

Question: I would like to know what metadata are saved and restored for files in iCloud drive. For example what kind of time stamps are kept (creation time, last read time, last modification time, ...)? Are file names case-sensitive? Are they normalised?

Has Apple published any reference on this?
iCloud Drive is not stored on a traditional file system per se on Apple's servers.

On your Mac, the files in iCloud Drive are stored simply on the existing file system on your hard drive - usually HFS+. You can find the files at the following path:

~/Library/Mobile\ Documents/com~apple~CloudDocs/