Three resolution sources, used in priority order
First, your device address book. Chat Hoarding for Android can export it as a vCard 3.0 (contacts.vcf) and bundle it alongside the backup file; the Mac app parses that vCard and stores the result inside the .tarc as contacts.json. Second, an adb-pulled or manually-imported CSV/vCard of contacts if you didn't use the companion (same downstream contacts.json). Third, manual mapping: tap an unknown JID inside the Mac app and assign a name; your edits land in a separate manual_contacts.json sidecar that wins over the auto-imported one. (WhatsApp's own contacts database, wa.db, lives inside /data/data/com.whatsapp on the phone and is not reachable without root, so Chat Hoarding does not read it.)