Can "tegnap" be placed at the beginning of the sentence too? "Tegnap hol voltál?"
Yes, that would be fine.
In Hungarian most of the word order is interchangeable, it is usually done for the purpose of emphasis.
This is possible in other languages to some extent, Hungarian tends to be a lot more flexible.
Word order is usually NOT interchangeable. There are changes that are nuanced and there are changes that change the connotation of the sentence completely.
Why not - "Hol tegnap voltál?"
Because we are asking about "hol". Meaning, the verb should be right after it.