This page says "user data" gets encrypted. But that implies it's only a certain part of the full disk? Like "/data" or something? Can I copy a file from a PC to the internal storage to any place and it gets encrypted? And what about an SD card, can I copy a file onto the SD card and it gets automatically encrypted?
It notes on the same page you referenced that a temporary filesystem is loaded into
/data, while the rest of the phone storage is encrypted. The purpose of this temporary file system is to perform the functions necessary to enable you to decrypt the phone, see a simple progress bar when decryption or to wipe the phone entirely. If you go to the section heading How Android full disk encryption works you can see this in further detail. To be safe, I would avoid use of the
/data folder to ensure all your necessary data is encrypted.
Encryption of external storage (such as an SD Card) will need to be handled manually. You can demonstrate this by turning an encrypted android device off and then plugging the SD card into your computer.
Also please see the disclaimer on the site which is important if you upgraded your device:
Caution: Devices upgraded to Android 5.0 and then encrypted may be returned to an unencrypted state by factory data reset.