I didn't really prep for residency and don't really recommend it, but some things that a good intern at a full scope program should at least be familiar with by the end of the yr ( not intangibles like social skills or being humble or making friends with the support staff)
Trauma- Dental/DA fractures (if you don't have GPR), mandible, NOE, ZMC, Le fort, ocular injury classifications and surgical approaches ( ex risdon, retromandibular, coronal, transconj, etc.) How to assess a patient
Soft tissue injuries - how to close lacs, types of sutures, suturing techniques
Infection- cellulitis, abscess, fascial spaces of the head and neck, common abx for different infx
Pathology-MRONJ vs ORN classification, common path like ameloblastoma, OKC, osteomyelitis, SCC staging and levels of neck dissection. Vein, artery, and soft tissue of common free flaps
TMJ-Wilkes, common treatments like conservative-arthroscopy/arthrocentesis-discectomy-TJR
Anesthesia- learn the basic MOA of common IV sedation drugs like propofol,ketamine, versed, fentanyl, precedex
Internal medicine- pretty broad but know your basic cardio,pulm, GI, kidney stuff at least
Orthognathic- basic concepts like what is a BSSRO vs IVRO, how to evaluate a pt clinically
Surgery- Know basic things like PEG vs G-tube, NG tubes, trach care, vent settings, SBO, all that good stuff
Nobody expects you to know anything, but if you have background info, when you see it in real life it's more likely to stick. Additionally, as an intern your main job is to report info to upper levels, not to do the surgeries themselves, but anytime you see a pt you should try to imagine what you would do if you were a chief and see how their treatment plans differ. Taking ownership of patients is how you learn.