China is ahead because of their advanced manufacturing economy. They literally build every single reactor component in house and don't have to really rely on any outside companies for basically anything.

They have mostly North American designed reactors, ranging from CANDUs to their version of the Westinghouse AP-1000. Westinghouse basically went bankrupt trying to make the AP-1000 a viable reactor and when they started slipping, China came in and bought the schematics, modified them, and that's their primary reactor design these days.

If you look at all the reactors worldwide that are being built, the majority of them in Europe are massively behind schedule for various reasons that probably get ignored by the Chinese. For example, one of the reactors in France is multiple billions above budget and about a decade behind due to the lack of precision of the builders, forcing them to re-assess and re-build key structural components multiple times over.