All defaults in a read-only format

If you just want to see what all the defaults are, use the Command Palette (Ctrl+Shift+P) and run "Preferences: Open Default Settings (JSON)". VSCode will generate a JSON description of all of the defaults.

Where the defaults come from

The default settings are hardcoded in the vscode sources.

Details

Let's look at some examples. When I open Settings, I see a long list, and this is at the top:

The first entry is "Files: Auto Save". That is defined by this fragment of Typescript code in files.contribution.ts:

        'files.autoSave': {
            'type': 'string',
            'enum': [AutoSaveConfiguration.OFF, AutoSaveConfiguration.AFTER_DELAY, AutoSaveConfiguration.ON_FOCUS_CHANGE, AutoSaveConfiguration.ON_WINDOW_CHANGE],
            'markdownEnumDescriptions': [
                nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'files.autoSave.off' }, "A dirty file is never automatically saved."),
                nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'files.autoSave.afterDelay' }, "A dirty file is automatically saved after the configured `#files.autoSaveDelay#`."),
                nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'files.autoSave.onFocusChange' }, "A dirty file is automatically saved when the editor loses focus."),
                nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'files.autoSave.onWindowChange' }, "A dirty file is automatically saved when the window loses focus.")
            ],
            'default': platform.isWeb ? AutoSaveConfiguration.AFTER_DELAY : AutoSaveConfiguration.OFF,
            'markdownDescription': nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'autoSave' }, "Controls auto save of dirty files. Read more about autosave here.", AutoSaveConfiguration.OFF, AutoSaveConfiguration.AFTER_DELAY, AutoSaveConfiguration.ON_FOCUS_CHANGE, AutoSaveConfiguration.ON_WINDOW_CHANGE, AutoSaveConfiguration.AFTER_DELAY)
        },

Notice the default value, which incidentally depends on the isWeb variable. Since I'm running VSCode on Windows (where isWeb is evidently false), I see a default value of "off" for this attribute.

The next attribute is "Files: Auto Save Delay". As it happens, the very next fragment in the same file contains it:

        'files.autoSaveDelay': {
            'type': 'number',
            'default': 1000,
            'markdownDescription': nls.localize({ comment: ['This is the description for a setting. Values surrounded by single quotes are not to be translated.'], key: 'autoSaveDelay' }, "Controls the delay in ms after which a dirty file is saved automatically. Only applies when `#files.autoSave#` is set to `{0}`.", AutoSaveConfiguration.AFTER_DELAY)
        },

Again, the default value of 1000 in the GUI comes from the default attribute here.

The next attribute is "Editor: Font Size". It comes from commonEditorConfig.ts:

        'editor.fontSize': {
            'type': 'number',
            'default': EDITOR_FONT_DEFAULTS.fontSize,
            'description': nls.localize('fontSize', "Controls the font size in pixels.")
        },

Here, the default value is not a literal, so we have to track down the definition of EDITOR_FONT_DEFAULTS.fontSize. Here it is, in editorOptions.ts:

export const EDITOR_FONT_DEFAULTS = {
    fontFamily: (
        platform.isMacintosh ? DEFAULT_MAC_FONT_FAMILY : (platform.isLinux ? DEFAULT_LINUX_FONT_FAMILY : DEFAULT_WINDOWS_FONT_FAMILY)
    ),
    fontWeight: 'normal',
    fontSize: (
        platform.isMacintosh ? 12 : 14
    ),
    lineHeight: 0,
    letterSpacing: 0,
};

It is again interesting that the default depends on the platform. Since I'm not running on Mac, I see a default of 14.

And so on. Each of the default settings comes from Typescript source code or, in some cases, package.json files for extensions (either built-in or installed by the user).

On the settings UI page, there's a special button in the top right corner gutter that toggles between the settings.json file and the settings UI.

Bonus Answer:

Add "workbench.settings.editor": "json" to your settings.json file. This will make it so that menu File → Preferences → Settings or ⌘ + , opens your settings.json file by default. Optionally, add a comment at the top to remind yourself where the toggle button is.

// Click the Open Settings (UI) button in the top
// right corner gutter to toggle the settings UI
{
    // Other settings in settings.json
    // ...
    "workbench.settings.editor": "json"
}

I had the same problem and the solution that worked is below. It is true that default settings cannot be modified, but the below steps explain how to edit the settings in JSON:

1. Cmd+Shift+P and select Open Settings UI (Ctrl+Shift+P for Windows I think)
2. Search for "Settings" and under "Workbench" click on Settings Editor. Change from "ui" to "json".
3. Search for “use split JSON” and tick “Controls whether to use split JSON…”.

Credit to Ahmad Awais video posted here