Nextcloud's E2EE is designed to protect user data from the server. The goal was to let users sync and share folders (and their contents) as easily as possible without the server ever having the ability to access the data. That rules out a web interface and has other limitations, but that's the price for knowing your server can't spy on you. It isn't and was never meant for ALL your data, as you should use a solution purely focused on E2EE instead! The nice features of Nextcloud, from online document editing to public sharing, will work on all your files except those you put in E2EE folders.

The talk is reasonably technical - it doesn't delve in to code or algorithms but sticks to the general design and flow of data: how do we create and share public keys, how do we get the private key on another device without the server being able to access it, how do we encrypt files and share them and so on.