Not sure about Android phones, but for iPhones, the setting is under the the WiFi settings. Settings > WiFi > Ask to Join Networks (these are outside your known networks). There are three settings for it Notify, Ask, and Off. Select "Off". You can also toggle WiFi on and off under the WiFi settings. Then your phone won't reach out to surrounding networks either. When you leave WiFi on, your phone is constantly searching for WiFi networks when you are not attached to your known network.
Another function to turn off is "Auto-Join Hotspots."
Also, turn off AirDrop unless you specifically want to use it (uses Bluetooth or cellular to move files between iPhones). With the newest iPhone software update, there is a new function called "Bringing Devices Together" that allows one iPhone to connect to another when setting it on top of another iPhone. It allow transfer of numbers with Name Drop. Or photos, etc. Turn off the AirDrop function unless you're wanting to specifically use it with someone. You can see the options within the AirDrop function. That is located under Settings > General > Air Drop
Thanks! Good info.
I have our Galaxy A53 set to only use home Xfinity wireless from a small modem, no Wifi, no hotspots, and yet I can stay connected for blocks in our city, even around a corner with a brick building in the way. Seems suspicious.