Just like on Windows where you can mount a network path to a drive letter (ex. Z:), in WSL2 it mounts Windows drives to a Unix path.
On your machine the C: drive was mapped to /mnt/c in WSL2.
So C:\dev\OCaml\RealWorldOCaml is at /mnt/c/dev/OCaml/RealWorldOCaml.
But a word of caution: These mounts can be 100X slower than a normal disk drive. When people complain about the speed of WSL2, the mount is almost always the culprit. Microsoft explains the details at Working across file systems | Microsoft Learn. TLDR: Move your files to /home/pyvovale/dev/OCaml/RealWorldOCaml (which is on a Linux ext4 filesystem) and then open that folder in VS Code.
(For future people who are just starting WSL2, you’ll need to learn how to move/copy/explore/change directories from the Linux command line. Try Linux Commands in more detail | High Performance Computing).