Google recently introduced the improved Nest Wifi mesh router system with integrated Assistant speakers. If you think that means you'll have to scramble to buy discontinued Google Wifi devices to expand your existing Google Wi-Fi network going forward, you'll be happy to learn that it's possible to use any Google Wifi and Nest Wifi devices in combination. You can add or replace some of your gear with Assistant-equipped access points.
When you already own a number of Google Wifis and only want to upgrade to the Nest Wifi router without its satellites, your best option is to make the latter your primary access point. To do that, you need to set up a new network via the Nest Wifi and add your Google Wifi devices as repeaters. You can also use the Nest Wifi router as a repeater, but Google doesn't recommend this option since it "doesn't take advantage of the improved range and speed" of the device.
Alternatively, if you only want to extend your existing Google Wifi set up with more access points, you can do it with the new Nest Wifi satellites and benefit from their integrated Assistant. Or you can use them as speakers only (but y tho?)
While OnHub devices work with Google Wifi routers, they're altogether incompatible with an infrastructure holding any Nest Wifi. Google says, "If your existing network contains an OnHub device, you’ll need to factory reset your devices and create a new network in the Google Home app" without the company's oldest router.