There are three major operators in China. Although most households use China Telecom's broadband Internet access, China Telecom's deployment and investment in the mobile terminal is far less than that of China Mobile and China Unicom. Most base stations of China Telecom's mobile terminal use the technology and equipment of China Unicom, especially the 4G network. Frankly speaking, it can be understood that telecom users give money to China Telecom and China Telecom gives part to China Unicom.
This is strategic cooperation, but it should be noted that Telecom 4G≠ Unicom 4G. Due to commercial monopoly and other reasons, although China Unicom and China Telecom both adopt FDD-LTE standard, China Unicom's mobile phone does not support telecom cards, and China Unicom's mobile phone does not support China Unicom's 4G (except the full netcom version).
The "dual 4G" mentioned by merchants means supporting Unicom 4G and mobile 4G, while telecom 4G only supports telecom mobile phones and full netcom versions.
Telecommunication equipment is made in Qualcomm, and Qualcomm occupies an absolute monopoly position in EV-DO network (i.e. telecommunication CDMA network). In the past, 4% telecom patent fee was charged (I don't know how much now), so mobile phone manufacturers can only wait for the price reduction of chips in Qualcomm before joining telecom standards, and it is reported that chip monopolist Qualcomm has definitely given up CDMA-4G research and development.