How can you optimize the website which has 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.

Hi Friends,

From an SEO point of view, for dynamic website, special additional SEO stuff has to be implemented.

Good Internal link structure
Generation of dynamic title and description
Dynamic XML sitemap generation

It depends upon architecture of your backend code plus your server setup.

First of all the set up your server plays a big role in speeding up thing. Having server like lighttpd add boost in compare to native apache. If you are serving good amount of static files then nothing is best than setting up reverse proxy with nginx to serve static files.
Whatever server you use don't forget to configure to properly according to type of traffic. Most of the time apache becomes pain just because of misconfiguration.
Just Google about setting up apache correctly and using reverse proxy to server static files (like images,css,videos etc)