Qualification:
I’ve heard various rumors, some of which seem to be true, such as that the phone works even when it’s turned off and maybe records all the time you’re talking, and with neural networks you can find out what you said, even if you were ten meters away from the phone and in another room, you can use the neural network to improve the sound quality and still know What did you say.
There is also information that the phone can see through objects, so gluing something to the camera will not help.
I don’t know what to do, I’m scared. Do you have any ideas on how to save myself from this if I live in the city and use my smartphone?
Of what? This is not a rhetorical question. Security starts with threat modeling, and your threat model dictates the precautions you need to take.
If you’re most people, your main privacy threat is advertisers and data brokers. Other comments have detailed how they collect data, and it’s usually “voluntary”. Defenses against this include a browser with good adblocking like Firefox with uBlock Origin, using websites instead of native apps as much as practical, using DNS-based adblocking, limiting or eliminating use of corporate social media, turning off voice-activated assistants, and preferring open source when practical.
It is not likely that advertising companies are activating the microphone or camera on your phone without your knowledge. The legal penalties for doing something like that in most countries would be ruinous for even the largest corporations, and the motivation for security researchers to check for things like that is substantial. If it did happen, the impact on your life would likely be a small payment from the resulting class-action lawsuit several years later.
If you live under a repressive regime that is known to routinely install spyware on phones, you may have different concerns. If an intelligence agency, large criminal organization, or multinational corporation is directly targeting you and willing to spend more money than most people have surveilling you, they’ll probably succeed even if you throw your phone in the ocean.