What web technologies to use for a big site?

Hi,
What would be the best choice to build a huge website upon? Should I use a framework or build a completely custom platform?

I'm planning to build a very complex learning management site where main functionalities would be: videos, audios, quizzes, chats, payments, messaging, users can interact with each other.

One of the development companies I contacted said they would use WordPress for this, but I'm not so sure it's a good idea.
The website will have a large user base (millions)...

What web technologies to use for a big site?

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