AFAIK till now it still uses Arial Unicode MS.
(The kanji parts of the Arial Unicode MS are terribly crafted by Monotype HongKong.)
On macOS, my recommendations are:
zh-Hans: PingFang SC
zh-Hant: PingFang TC
zh-Hant-HK: PingFang HK
ja: Hiragino Kaku Gothic W3
ko: Apple SD Gothic Neo
zh-CN: Microsoft YaHei
zh-TW & zh-HK: Microsoft JhengHei
ko: Malgun Gothic
P.S.: on both platforms plz forget about Yu Gothic which also has legibility issues when being used in UI font size.