Google's cellular service, known as Google Fi, is a unique cellular provider since it utilizes three different networks (Sprint, T-Mobile and US Cellular) as well as Wi-Fi behind the scenes - automatically picking the best wherever you happen to be.
To take advantage of this feature, however, requires a special "designed for Fi" phone with the hardware necessary to handle this switching.
Switching between cellular networks, however, always came with a delay that would interrupt your connection - annoying at best.
But the next generation of Fi phones, beginning with the Pixel 4 (released in October 2019) will utilize a technology called Dual-Sim Dual Standby (DSDS) to eliminate this interruption when the phone switches between cellular providers in the background.
DSDS works like this: The phone selects one primary network for sending data or making calls, but a secondary network is kept active and available to switch to instantly. This allows for seamless network switching and also allows for more frequent switching. To use DSDS to connect to two networks simultaneously, made-for-Fi phones use a standard SIM card and the built-in eSIM at the same time.
Google Fi plans to roll out and enable this new feature to Pixel 4 phones in late October and November 2019 and promises to support future made-for-Fi phones that incorporate DSDS technology.