Learn Portuguese (Portugal): Alguém está à porta.

We use à (the contraction of the preposition a + the article a) when talking about being near or waiting at a threshold, like 'at the door' or 'at the window'. If you used na (which means 'in' or 'on' the), it would sound like the person is physically attached to the wood of the door, like a sign or a doorknob!

I was taught estar is for temporary things. Why use está here?

The 'temporary vs. permanent' rule is just a beginner shortcut that often fails. The better rule to remember is that we almost always use estar to describe the physical location of a person or a thing. You only use ser for location when talking about where an event takes place (like a party or a meeting).

Does the word alguém always use the 'he/she/it' verb form?

Yes. Alguém means 'someone' or 'anybody', and just like in English, it always takes the third-person singular form of the verb.