What is in `/usr/bin/python3`?

I'm on macOS Sequoia 15.3.2, and I've been experiencing some weird behavior with /usr/bin/python3 lately. Usually it seems like a perfectly normal Python 3.9 binary:

$ /usr/bin/python3 --version
Python 3.9.6

But then when I cd into a directory with [use flake](https://github.com/nix-community/nix-direnv) in its [.envrc](https://direnv.net/) file, it suddenly starts pretending like it doesn't exist:

$ /usr/bin/python3 --version
error: tool 'python3' not found

But the _file_ does exist:

$ stat /usr/bin/python3
  File: /usr/bin/python3
  Size: 118848    	Blocks: 24         IO Block: 4096   regular file
Device: 1,14	Inode: 1152921500312496951  Links: 78
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/   wheel)
Access: 2025-03-06 05:06:02.000000000 -0500
Modify: 2025-03-06 05:06:02.000000000 -0500
Change: 2025-03-06 05:06:02.000000000 -0500
 Birth: 2025-03-06 05:06:02.000000000 -0500

Curiously, its first 4 bytes are 0xCAFEBABE, which I thought [was associated with Java](https://stackoverflow.com/a/2808684/5044950) :

$ xxd /usr/bin/python3 | head
00000000: cafe babe 0000 0002 0100 0007 0000 0003  ................
00000010: 0000 4000 0000 7f20 0000 000e 0100 000c  ..@.... ........
00000020: 8000 0002 0000 c000 0001 1040 0000 000e  ...........@....
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000080: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000090: 0000 0000 0000 0000 0000 0000 0000 0000  ................

But, naively, I don't see "Java" mentioned anywhere in the binary; that is, this command returns nothing:

$ strings /usr/bin/python3 | grep -i java

**What is in /usr/bin/python3 on macOS?** What kind of binary is it, and why does it print this bizarre "tool 'python3' not found" error message in some situations?