What web technologies to use for a large site?

What would be the best option to build a huge website? Should I use a framework or build a completely customized platform?

I am planning to build a very complex learning management site where the main functionalities would be: videos, audios, tests, chats, payments, messages, users can interact with each other.

One of the development companies that I contacted said that they would use WordPress for this, but I'm not sure it's a good idea.
The website will have a large user base (millions) very quickly and I hope it is very busy. I'm not sure that WordPress can handle such large traffic and user activity.

So, what kind of technologies or framework would you recommend?

Thank you