This has nothing to do with singular/plural and everything to do with verb conjugation, mostly because like, as it's used here is a verb, not a noun.
- "He likes to watch movies"
- Third person singular present tense.
- "They like to watch movies"
- Third person plural present tense.
- You like
- Second person present tense
- "He doesn't like to watch movies"
- "He does like to watch movies"
- "Does he like to watch movies?"
- All three of the above are infinitive.
Use-mention distinction
This is an example of the use-mention distinction. In the sentence "Who is they [sic].", "they" isn't being used as a pronoun, but instead is being used a word. That is, it's referring back to the use of the word "they" in the previous sentence ("Now they've decided to take on Ava Hessington personally.")
Notice in that sentence by A the "they" takes the plural form of the verb ("they are"/"they're"). This is always the case with "they" when it's used as a pronoun; it's always gramatically plural, even when it's used to refer to a single person. (Which, despite moaning by some pedants, is a perfectly valid English construct.)
In the sentence by B ("Who is they?"), the "they" isn't being used as a pronoun, but is being used to refer to the word "they" that was used by A in the previous sentence. The convention in English is that when you have this sort of "mention" case - where you use a word as a word as opposed to using it for its meaning - you should place that word in quotes or italics. So the closed captioner should really have rendered it as
Who is "they"?
or
Who is they?
The fact that they didn't is either due to limitations of the closed captioning system (for example an inability of the system they were using to represent quotes or italics), or simply because the captioner wasn't aware it was needed.
Note that the quotes/italics here aren't being used as "scare quotes" or as some sort of marking of incorrect usage, but as a literal quote of the word itself. As such, the speaker wouldn't actually make an air quote gesture, but probably would subtly emphasize the "they", either by saying it in a slightly different tone or pausing slightly before saying it.
Best Answer
This is an example of "singular they". Here it is being used, apparently, to avoid the need to use a gendered pronoun to refer ro a person of unknown or unspecified gender.
This is now a common usage. It has been used by respected writers of English since the 14th century, but is now much more common than it used to be. A few decades ago this sentence would have been likely rendered as :