Recorded in different tracks. And there are a lot of copies of tracks with back vocals. Comes to 8-16 of each party. In addition, they record tracks with subtone, octave up and down, as well as whispers and preferably different timbres.
In general, recording back vocals is no less difficult task than recording the main voice. Not every vocalist can "repeat" all the features of a soloist that is required for high-quality backing vocals.
Of course in tracks. That's why he is "back", back. First, the main vocals, which are made loudest. The rest of the tracks add backing vocals, perhaps even with various effects, such as a wave.