Google Chrome uses Blink rendering engine, while Safari uses WebKit.
But, in iOS, Google Chrome uses WebKit. My question is, does Google Chrome macOS client uses WebKit rendering engine too?
No. While Google Chrome used WebKit for macOS client at one point, that's no longer the case for current stable build. From the Wikipedia entry for Google Chrome:
WebKit was the original rendering engine, but Google eventually forked it to create the Blink engine; all Chrome variants except iOS now use Blink
The restriction to use WebKit as the rendering engine for 3rd party Web browser apps exists solely on iOS. From the App Store Review Guidelines: