Learn French: Le documentaire dont elle se souvient toujours est intéressant. - The documentary she always remembers is interesting.

The verb "to remember" is se souvenir de in French. Because it requires the preposition de, we must use the relative pronoun dont instead of que to link the ideas.

Where does the de go from se souvenir de?

The word dont already includes the meaning of de inside it. Since dont replaces de plus the thing being remembered (le documentaire), you drop the de completely from the rest of the clause.

Why is the word order se souvient toujours instead of toujours se souvient like in English?

In English, we put "always" before the verb ("always remembers"). In French, adverbs like toujours usually go immediately after the conjugated verb, giving us se souvient toujours.

Could I use se rappeler (to recall) instead of se souvenir here?

You could, but it changes the grammar! The verb se rappeler usually takes a direct object without de. If you used it, you would have to drop dont and use que: Le documentaire qu'elle se rappelle toujours est intéressant. This is a great way to see why dont is strictly tied to verbs using de.