Olivia and Hugh both say the phrase "Journeys end in lovers meeting." This is a quote from William Shakespeare's "Twelfth Night" (Act II, scene III). This is more directly a reference, however, to the line's repeated usage in the original book, where Nell uses it as a mantra and driving force.
A picture of a blue butterfly with no antennae is on the wall of the hotel room Luke ( Oliver Jackson-Cohen) and Joey ( Anna Enger Ritch) stay in. Blue butterflies without antennae feature in Mike Flanagan's previous film Before I Wake (2016).
The final song is If I Go, I'm Goin by Gregory Alan Isakov
When Steven is following young Hugh into the room with the spiral staircase, there is a ghost standing in the window, halfway behind a corner.
The episode's title "Silence Lay Steadily" is taken from the paragraph that opens the book, and also closes out the book, the show, and The Haunting (1963). "silence lay steadily against the wood and stone of hill House, and whatever walked there, walked alone."