If I want to say "these things have been studied" would I use passive voice, as in "se han estudiado?"
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"Han sido estudiadas" is the true passive voice.
"Se han estudiado" is reflexive.
Spanish allows to utilize two different types of passive sentences, so it would have been possible to make both of the next translations:
The first option is normally chosen when the subject is important (estas cosas), while the second is used when is not, and is what happens what is stressed in your sentence.