When people say “they're engaged,” the phrase “to be married” 'is understood'. [=“they're engaged” is understood to mean “they're engaged to be married”]
We still don't fully 'understand' the causes of the disease. = The causes of the disease are still not fully 'understood'.
He believed it 'was understood' [=believed that everyone had agreed] that he would be paid for the work.
(..) - He was nominated for an Oscar. - Why? - But, sadly, he recently died. - How? - He choked himself off. That's the first thing I understood. Go ahead. Okay. "God..." - Good. - "Good movement..." - Morning. (..)2020 TvShow: Family Guy Title: Movin' In (Principal Shepherd's Song) Season: 18 Episode 20