If Android says it's connected to a WiFi network, does that mean it's not using any mobile data? Could it ever happen that somehow an apps insists on using mobile data despite having a WiFi connection? In other words, is the only way to guarantee not to be using mobile data is to disable it?
For context, I noticed Google Photos doesn't sync even though it is set to. If I disable "only sync over WiFi" then it works, even if I am connected to WiFi and have mobile data off. Is it necessary to turn off mobile data to make sure it's not used?