These works are Vocaloid works, which are vocal-covered by VOCALOID. 
Vocaloid is a vocal-synthesizer developed by YAMAHA.  
Various of sound banks are used. 
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Chinese sound banks: V3 TianYi Luo/V3 He Yan
Japanese sound banks: V3 Hatsune Miku(original ver.)/ V3 Megurine Luka
English sound bank: V3 Kaito-eng.
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Most of those works are processed by 3 procedures: Vocaloid dry sound output-----(Adobe Audition used) reverb and after effects, EQ, enhancer, high-pass filter,Analog Echo, and a variety of VST plugins. 
-----(Izotope Ozone7 used)mastering.

Umiumip 海歌