If you want to create your own theme, rather than extend an existing theme.
Either manually create 5 files in a new folder in your themes folder:
or download theme-experiments and run the create-empty-theme.php routine.
or follow the instructions at
https://github.com/WordPress/gutenberg/blob/trunk/docs/how-to-guides/themes/create-block-theme.md
This used to work…
https://developer.wordpress.org/block-editor/how-to-guides/block-based-themes/
This does… but it’s out of date.