Anyone who's used a traditional WiFi extender will know that you sometimes need to manually switch your devices to the extender's own WiFi network when you're too far from your main WiFi router. That's mostly because WiFi extenders have their own network name that's slightly different to your main WiFi network's name. For example, an extender's network name might be your main network's name with the letters "EXT" tagged on, to indicate that it's your extender's WiFi network.
Mesh systems like Eero, on the other hand, help your devices automatically switch to the nearest Eero, and each Eero broadcasts the same WiFi network name.
Eero will also automatically connect your devices to the appropriate WiFi band — 2.4GHz and 5Ghz — depending on their needs. If your devices need better range, Eero will automatically connect your devices to the longer-range 2.4GHz band. If you're close enough to an Eero router, it'll connect your devices to the 5GHz band, which offers faster speeds, but has less range than the 2.4GHz band.
To be fair, some recent extenders give options to keep your main WiFi network's name and bands to make switching easier and automatic. At the same time, you might need to go into the extender's settings to enable that feature, which can be complicated for a lot of users.