The HyperCard stack's resource fork contains icons, sounds, xcmds, xfcns, and probably some other miscellanea. Most of the stack is stored in the data fork.

APFS doesn't specifically have resource forks. They're handled as the com.apple.ResourceFork extended attribute (which is also one way OS X exposes them on HFS+ but with HFS+ the fork/xattr always exists; on APFS it has to be explicitly created).