One option is to use a feature that converts one or more notes to a Google Docs document.
1. Select all your notes: go to Google Keep and press Ctrl+A (or Cmd-A for Mac).
Important: This only selects the notes from the current view, so archived notes aren't included. You can repeat these steps for archived notes or select all your archived notes and unarchive them.
2. Click the 3-dot icon from the top of the page and pick "Copy to Google Doc".
3. Wait a few seconds and you should see a link at the bottom of the page that says: "Open doc". Click that link to open the document that includes all your notes.
Another option is to use Google Takeout and export all your notes as HTML files. Google Takeout exports all your notes, including archived notes and notes from the Trash.
You'll get a ZIP archive with HTML files for each note. The archive may also includes image and audio files. If a note doesn't have a title, the exported HTML file will use the date in the filename.