Every router is different, but mine has the option where you can set rules. Its default was set to "allow all new devices to connect", but I changed it to block new devices from connecting. So after you verify all wired and wireless connections are legitimate, i.e. that they are yours, you can turn on the block mode so that no new devices can connect. It is a bit annoying when I add something new in my house, but this prevents any new device, particularly a foreign one, from connecting to my network.
I used to design IC's (integrated circuits) for multiple wireless technologies (cellular, GPS, WiFi, BT, etc.). One thing to keep in mind is that most radios are a significant battery drain, so there are a multitude of different sleep and deep sleep modes (DSM). When a device is powered "on", it rarely, if ever goes into DSM. However while in sleep mode, it is usually permitted for the radio to power on, see what signals are present and power back down. This is a must in GPS where ephemeris data needs to be updated to minimize the time to first fix (TTFF). Without it, you would have to wait 5-10 minutes for the GPS to figure out where you are before you could navigate anywhere. It is for different reasons, but BT and WiFi similarly have provisions in their protocols where they are allowed to wake up and monitor the radio environment. My point here is just because you have turned off the WiFi switch on the side of your laptop, don't assume that the radios never turn on because most likely the SW and HW are designed for them to turn themselves on. Really DSM and true off (battery unplugged or cord removed) are about the only times that you can guarantee radios are off in electronics today. Good example of this is the iPhone where customers thought airplane mode meant cellular, WiFi, BT, GPS, etc. all turned off, but in reality airplane mode only turns off the cellular modem, leaving the user and the OS free to turn on the other radios.
With regard to a device such as a smart tv connecting to an unsecure WiFi network on its own - Very unlikely any device would direct connect to a network without user control....MOST of the time. Perfect example of an exception is Apple and Google's plan for contact tracing to help in this pandemic...it would have your phone turn its BT on throughout the day and contact other BT devices nearby to see if anyone is COVID-19 positive within BT range of your phone. All without the user being aware of this.
With regard to devices phoning home and/or being exploited for camera spying, etc., it has happened. When all of these devices became smart, e.g. thermostats, doorbells, etc., they did so with low end microcontrollers with limited to no security. This has made them a prime target for hackers. In addition to this you have state level actors like China putting backdoors in devices with their IC's in them. Anyone remember the Clipper chip that our government was pushing before? It has been a few years, but I remember Lenovo getting singled out previously for shipping brand new laptops with Chinese spyware on them. I believe there was a drone maker that got caught with their devices sending some kind of "data" back to servers in China. I have also seen reports that the Ring doorbell has been compromised for a few users. As devices get smarter, oftentimes they will send back data to corporate HQ as part of an analytics program to improve future products...so not all devices phoning home are doing so with ill intent.
While it is unlikely the average person will ever get legitimately hacked, it is possible to fall victim to spyware and/or purchase a compromised electronic device putting you "at risk" without knowing it.
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