We are trying to implement hreflang on multilingual ecommerce site which has 800K + URLs . We have below questions

1. Is hreflang implementation a manual process: Example: Developers/SEO team will identify products that are available on all the stores and update the hreflang store links.?
2. Is there a tool that can help develop hreflang based on available products? (We are not using any CMS Platform)
3. How the technical implementation works for larger sites with any URLs in it.