Your exported file is fully standalone — drop it anywhere static files are served.

• GitHub Pages: add 404.html to the repo root. It auto-serves on any missing path.
• Netlify: place 404.html in your publish directory, or set [[redirects]] for custom statuses.
• Vercel: add 404.html / 500.html at the output root.
• nginx: error_page 404 /404.html; then a matching location block.
• Apache: ErrorDocument 404 /404.html in .htaccess.

Informational only — nothing is uploaded anywhere.