I believe that content of “Preferences: Default Keyboard Shortcuts (JSON)” (command ID workbench.action.openDefaultKeybindingsFile) really shows comprehensive list of all native and extensions-contributed commands VSC knows about at moment when invoked.
This file shows keys from VSC’s defaults and extension manifests.
Commands with no suggested defaults are those commented out at the end of file.
Their descriptions (as seen in the Command Palette, Keyboard Shortcuts settings, extension Contributions tab and elsewhere) are supposedly in localization properties and I believe there is currently no way to see them along their respective command IDs in single convenient “localized” list. So for now the only way to read the description of command found in aforementioned JSON is pasting its ID into Keyboard Shortcuts search field. (Would be delighted to be proven wrong.)