the pianos are synthesized.

there is a decent amount of slight pitch modulation that increases in randomness with sustain. the velocity of each note is also varied which leads to the different sound of each note as they are played. probably a small amount of tremolo is being applied as well.

definitely a bit of unison going on as the higher lead comes in at around 00:21, creating that warm boards of canada sound. its not a super complicated sound but you will need a bit of fucking around with waveshapes if you are trying to create the exact tone.

a really big thing is setting the mix up correctly. the lower sustained notes have a tiny bit of phase sync/position spread unison causing each ear to be playing slightly differently tuned sounds and are mixed quite loud in comparison to the higher lead which is may be mixed mono coated in a bit more reverb giving it a feeling of separation or floating along in the atmosphere created by the lower notes.

@00:57 theres a pad that comes in with a similar combo of detuning/unison and reverb with perhaps a bit of tremolo and a slow attack. some type of low passed triangle ish waveform and multiple voices. @ the same time there is a low passed bass synth pluck that is mixed to be almost panning around the head. probably a square wave with a low pass filter that is attached to a adsr w a hint more attack than the actual synth. lots of unison/multiple voices as well.

when the "And I"s comes in there's a lot of goodies happening with the different reverbs of each sound with some of them like the vocals pretty harshly cutting out & a bit of feedback looping on others. there's also a bit of analog sounding synth atmosphere sounds going on faintly in the back. could be like a additive granular sound.

the really great percussive sounds @ 1:40 during the "all wrapped in cellophane" part is definitely some type of additive/granular synth processing on a couple different percussive sounds with probably 4 variations: a metallic transient one, a slightly slower hat sound, a quite fast hat sound and the little shaker sounding one. My best guess is that the samples being used for the additive synthesis are drum sounds.

the big hook afterwards has a pretty fat kick coming in sounds like it might have a bit of feedback on a small loose reverb and the bass sound is mostly a standard 808 sound with a tremolo.

the clacking percussive noise during the next "and I" section is probably using the same technique as the earlier percussive sounds just with a different sample.

in my opinion the truly gorgeous thing about the production is about how the disparate elements are composed and mixed together to create somewhat shocking and gorgeous sections that are constantly evolving @ every step of the song to provide the power and gravitas to the songwriting and vocals. the overall affect of the various production techniques are to create a kind of rift between the analog synths & sound design feeling like "home" while the alien granular elements provide a cold counterpoint. mirroring the themes of the song it creates a grand feeling of distance and alienation to the feeling of hominess and comfort. also its possible that whatever sound is used for the epiano is also granular synthesis of a epiano sound. and ofc her vocal performance is amazing too.