Open WSL folders in Explorer and the other way around

From a WSL shell, open the current folder in Windows Explorer:

explorer.exe .

Any Windows program works that way: code . opens VS Code, notepad.exe file.txt does what you'd expect.

From Windows, the Linux filesystem is a network path. Type it in Explorer's address bar or pin it:

\\wsl$\Ubuntu\home\ian

And your Windows drives are mounted inside WSL under /mnt:

cd /mnt/c/Users/ian/Downloads

Keep project files on the Linux side (~/projects), not under /mnt/c. File access across the boundary is much slower, and npm install on /mnt/c is painful.