How difficult it is to find out someone's searching patterns on Google for example or sites I'm visiting? There is things like disconnect search engine, Tor browser and VPN etc. that provides the anonymity and privacy and it seems to be a growing concern for active internet users today. Can someone find out my search behaviour through IP adress for example, through my ISP or something similar?
One of the most useful explanations I've found is the EFF's page on Tor and HTTPS. It's an interactive diagram that shows who has access to what information about you, normally, when using one of Tor or HTTPS, or using both. It is a bit U.S.-centric; laws in other countries may affect what organizations can legally gather.
One limitation to what that page covers is that it only discusses what an observer can find out about a single HTTP request. When discussing things like tracking your search results across multiple days, there's more to talk about in terms of deanonymizing traffic. That is, if I know someone searched for "foo" and I know someone searched for "bar", do I know those were the same person? IP is the most obvious way you can be tracked, and persistent cookies is another well-known option, but there are also plenty of more sophisticated options like HSTS super cookies and fingerprinting (more here and here). It's even easier if you log in to sites (Are you logged into a Google account? A Facebook account?).
There's a lot more to talk about, more than will probably fit in a Stack Exchange answer. If you have more specific questions, I'd encourage you to ask those separately.
The very short tldr is "yes, companies and governments (yours and others) know what you're doing".