Where is the 'launch.json' file in Visual Studio Code?

As described in the Launch Configurations section of the Visual Studio Code documentation:

VS Code keeps debugging configuration information in a launch.json file located in a .vscode folder in your workspace (project root folder) or in your user settings or workspace settings.

To create a launch.json file, click the create a launch.json file link in the Run start view.

As of Visual Studio Code 1.56:

Once that's created, it should now be available under your workspace's .vscode folder.

Click on the debug side bar to open the debugging options. Then either the cog icon at the top, or the create a launch.json file link below the big blue button. If the launch.json file is present, it will open it otherwise it will generate one.

Warning: if you try to create an empty one (or one with an invalid content), trying to get to that file through the "Create a launch.json file" would not do anything.

See issue 133327.

That will be fixed with VSCode 1.61 (Sept. 2021): at least, that existing file will be opened when you click on "Create a launch.json file".