You’re incorrect as to the US, and I suspect as to Japan as well. Lots of criminals in America plead guilty, but that doesn’t mean that there’s no “external proof” of guilt. Cops are lazy and criminals are dumb. So what you end up with in nearly all cases is that cops make arrests in easy cases with tons of evidence. Confronted with the video tape, cell phone records, etc., the accused then pleads guilty. Rarely do you see the kind of cases you hear about on Podcasts where guilt turns on circumstantial evidences and confessions. I’ve seen this from inside the court system.
My wife had a homicide trial where prosecutors had a complete video tape record of the accused shooting the victim and driving back to his house. The trial was just cutting from one CCTV feed to the next as the accused drove around town. The jury found him guilty—because of course they did.