You read books and publications particularly those published by the professional institutions. You attend conferences and seminars on your speciality subject. Maybe the company you work for organises training sessions for you on engineering, commercial or management topics. You may establish contacts in the academic world which uaually welcomes cooperation with industry (at least in engineering). You study and evaluate the competitions' technology, literature and pricing. You have plenty of discourse with colleagues.and customers.
If you become complacent, get it wrong or slip behind, you can end up like Nokia in the aftermath of the i-phone.
You don't. It has been a hundred years or more since anyone could claim to "know it all". Even then it was doubtful such a claim was true. A wise man once said that every fool knows something you don't. The internet is a wonderful resource, unfortunately is also full of nonsense so you really have to have a good understanding of math and science to separate real science from fiction and pseudoscience. Even respectable media companies publish nonsense because they are primarily entertainment and use incompetents for consultants.
Magazines, seminars, technical publications, word of mouth and technical sales people.
We read stuff. And due to long experience we don't need huge explanations to understand stuff.
Not only considering the technology (yes this too) but even the innovative ideas or even marketing techniques.
