The narrator is a 3rd-person narrator. This can be seen for example at the start of chapter 10 where it is explained that “[d]uring the night the nameplate of each new Twelve had been removed by the Maintenance Crew and replaced with the style that indicated citizen-in-training”, which isn’t something Jonas or Fiona say but probably still knows.
A third person narrator allows giving a limited amount of information to the reader otherwise inaccessible to the characters to support the story and underline the characters behaviour.
a) “Then she [the attendant at the Annex] seemed to notice his discomfort and to realize its origin. No doors in the community were locked, ever. None that Jonas knew of, anyway.” - The fact that the doors to the Annex were locked makes the place and what happens inside more mysterious and occult.
b) “But this room’s walls were completely covered by bookcases, filled, which reached to the ceiling. There must have been hundreds - perhaps thousands […].” - This makes The Giver and The Annex seem very special as they hold a lot of wisdom & information.