Intro: A parody of "The Omen," with Maggie as a demon baby. Not bad. Also it ends with a fun reference to the fact that this is The Simpsons' 666th episode.
Danger Things: A parody of Stranger Things, with Milhouse being abducted into the "Under-Over," and Lisa taking the role of Eleven, to try and rescue him. In general, I enjoyed this whole bit, and all the 80s references (like E.T. the Atari 2600 game, and Tracey Ullman-era Simpsons).
Heaven Swipes Right: A parody of "Heaven Can Wait," with Homer dying before his time, and being sent back to Earth to possess a series of different people's bodies. This segment was just sort of okay.
When Hairy Met Slimy: Despite the title being a play on When Harry Met Sally, this segment is actually a parody of "The Shape of Water." Selma falls in love with Kang, and eventually enlists Homer's help to break him out of the secret facility where Mr. Burns is keeping him, to meet up with Kodos and escape from Earth. It definitely had some good gags and some terrible ones, but on the whole I liked it, mainly because it's just nice to see Kang and Kodos having a major role in "Treehouse of Horror" again, after a number of years being either completely absent or having only minor cameo appearances.