The origin of "imaginary" in the art creation of Chinese garden is closely connected to Chinese philosophical ideas.
In this piece I see a Chinese garden with a pool, bridges and mountains in the distance. This imagery is of paradise of idyllic beauty and tranquillity, this is Shangri-la, another word for which is Xianggelila.
Chinese gardens aren’t laid out in a way that you can see the entire garden all at once. Instead, small scenes are set up so that as you wander through the garden, you come upon several intimate settings to view. Every scene is well-planned and framed.
When I play this piece I see it as a journey through a Chinese garden with small scenes, ending with a temple to the ancestors.
Originally from Beijing, China, Zhang Nan is a Los Angeles-based composer for film and television. She started playing piano at four years old and composed her first piano piece at age five. This strong enthusiasm in music led her to step away from the stable role of a recording engineer in China Central Television to become an independent composer. 
This work deftly balances elements of the pentatonic melody inspired by traditional Chinese music and imagery, and the more familiar Western harmony. The writing contains many elements that the instrument excels at, while maintaining a pentatonic foundation.
I do not believe you can discuss the form and timbre of a Chinese piece without including symbology as it is interwoven into the history of pentatonic intonations.
This piece has three distinct sections without any firm separation of clear cadential points. The first section sets the scene with a B major tonality and a pentatonic from B using the notes B, C#, D#, F# and G#.
The melody is clear with a guqin character with strong pentatonic resonance. The first section culminates with an imperfect cadence moving from a subdominant chord E9 (E, G#, B, F#) in the first part of bar sixteen progressing to a dominant seventh chord (F#, A#, C#, D#) in the second half of the bar concluding at the start of bar seventeen then joining seamlessly to the second section.
The second part reminds me of a Chinese garden bridge both in sound and length.
In China water is important and symbolises communication and dreams .
This may be interpreted as water which is considered to relate to emotions and air pertains to intellect. In this middle section the melody is the bridge over the water, a combination of intellect rising above and over dreams. The melody in the right hand is played from C6 rising to C7, this pitch symbolises both air and, potentially, moonlight.
The left hand melody, symbolising water, has two bars using the notes from a pentatonic on B, a single bar of a pentatonic on C sharp, then returns to two further bars from a pentatonic on B. This ebb and flow of tonalities supports the concept of water. Culminating this section, and flowing into the next, is a pentatonic on the note E also known as the subdominant.
In the third section there is a feeling of ascension throughout. The four bar melody in the right hand is repeated with a variation of the foundation harmonies culminating in a tempo rubato and four significant chords, three based on B pentatonic and one on C sharp pentatonic heralding the arrival of the most poignant and significant chords of the piece, descending in the bass. These chords are the dominant, the subdominant and the relative minor of the tonic by the key note, not by the key signature. An interesting inclusion on the third chord is the seventh note from the descending melodic minor scale. This follows the line from the previous two notes and is left in uncertainty with the increased tension on the crescendo and the pause on the minim. Finally the ascending grace notes from a C sharp pentatonic scale rise into a glorious tonic chord.
Between the communicable Tao and incommunicable Tao, the mind, and the heart, and the voice, and the hand there are three bridges we must navigate: the bridge of the inner voice, the bridge of mutual understanding between speaker and listener, and the bridge of language itself. When the hands, heart and mind are unified, the inner voice speaks. 
The bridge of the inner voice.
This is the first section. The inner voice is that of the child, no thought, no reason, just pure sparkling sound.
The bridge of mutual understanding between speaker and listener.
This is the second section, with the speaker is one hand, the listener is the other and the mutual understanding is the unspoken continuous connection.
The bridge of language.
Language is more than the spoken word. Should the understanding have been achieved the ending will bring peace, should it have been misunderstood, likewise the minor chord will be confusing.
The above quote states “When the hands, heart and mind are unified, the inner voice speaks.” When the performance includes hand, heart and mind, the listener should hear the inner voice speak and understand, words will not be required. This is symbolised by the final ascending chords, as pure understanding blossoms into the final statement of the pentatonic chord.
My image of Shangri-La can be found by searching for Shangri-la fantasy mythical. This image is from Reddit and claims to be Auridon from the game Elder Scrolls Online. I have also found it on various wall paper sites.
 刘明. “The Art of Chinese Garden and Traditional Chinese Culture - Chinadaily.Com.Cn.” Www.Chinadaily.Com.Cn, 2 Sept. 2018, www.chinadaily.com.cn/a/201812/09/WS5ca315dfa3104842260b3fb5.html. Accessed 17 June 2020. The Humble Administrator's Garden, a renowned Chinese garden in Suzhou. [Photo provided to China Daily]
 Ruru Zhou. “Features of Chinese Gardens.” China Highlights, 2019, www.chinahighlights.com/travelguide/architecture/features-garden.htm
 Chia, Mantak, and Tao Huang. The Secret Teachings of the Tao Te Ching. www.Scribd.com, Bear and Company, 31 Jan. 2005, pp. 29–30, www.scribd.com/read/351488090/The-Secret-Teachings-of-the-Tao-Te-Ching. Accessed 17 June 2020.
 “NanZhang Music.” NanZhang Music, www.nanzhangmusic.com/. Accessed 17 June 2020.