Tower Controller reported a military training aircraft pilot called them after landing to report a NMAC while they were in the TRACON's airspace.

Narrative

Aircraft X landed at ZZZ. The pilot of Aircraft X contacted ZZZ Tower on a recorded line and reported that they had a near midair when they were inbound to ZZZ. Aircraft X reported at XA:15Z they were talking to ZZZ Approach and were told to expect vectors for the LOC RWY XXL at ZZZ. A VFR aircraft that was not talking to ZZZ Approach or ZZZ Tower was transitioning to the west; north of ZZZ Class D airspace. Aircraft X reported the VFR aircraft passed less than 500ft directly below them. Aircraft X reported that the traffic was not issued. Aircraft X reported they responded to a TCAS RA to climb and climbed from 024 to 027 and the TCAS RA was resolved. Aircraft X continued inbound visually and landed without further incident. Aircraft X reported that the VFR aircraft was Aircraft Y; and this information was from ADS-B.Recommendation: The area near ZZZZZ continues to be a dangerous area with multiple aircraft crossing this congested airspace without talking to air traffic. The airspace is outdated and need updates to include ZZZZZ the FAF for RWY XXL.