You can have a computer that does not connect to the Internet, or connects to the Internet very little, or only connects to the Internet through specific communication channels you open in a firewall. That's all very attainable.
However, as soon as you communicate with third parties, be it your ISP, a cloud provider, or your end communication partner, you are potentially sharing with more than you intend. It's a "the only way to keep a secret between three people is if two of them are dead" problem. You can't control what other people choose to share.
