How do I find the font file named 'Heiti TC' on my mac?
Heiti TC is built into macOS.
You should be able to see it in Font Book, from where you can reveal in Finder at /System/Library/Fonts/STHeiti Light.ttc [also medium]
List of included fonts - Apple KB - Fonts included with macOS Catalina [was included long before Catalina, but I don't know from when]