How to optimize a website with millions of pages?
Yes, you can do this.
You need strong backend and architecture for your website. There are some paid tools which handle SEO of million pages.
But, you need to check them all.
You can check website errors in Google search console and screaming frog.
You need a big team for handling these million pages website.
You need to start with the technical audit.
update your content at least once per week. Add Image and video in pages – Not only search engines but user also love to watch graphical and video related content. Add related rich media content to optimize your web pages better for user as well as search engine.
Analyze all of your website data,Conduct thorough keyword research,Produce long and value-rich content,Optimize for on-page SEO,Optimize for off-page SEO,Optimize the website for mobile,Speed up the pages and Get quality backlinks.