Often people will talk about complexity and complicated things as if they are the same thing. They are not, and the difference is important. Once you understand the difference you can find many useful applications to your own thinking and actions.
‘Complex’ as an adjective describes complexity. It can be defined as “having parts that go together in complicated ways.” They key word is ‘parts.’ It may have several relatively simple components that fit together. The system as a whole, that is to say how they interact and behave together, can be what it makes it complex. Indeed, if you own an “apartment complex” then it consists of several units that may or may not require integrated heating, plumbing, and electrical service. Each apartment on its own is relatively straightforward but taken as a whole it becomes complex. One tenant may use all the hot water and that affects everyone else. Another tenant may not pay his rent on time and that affects the management system that maintains and keeps the system healthy in the first place. If a good manager understands all the components well, the apartment complex may still be complex but he can keep it functioning.
‘Complicated,’ in contrast, is hard to understand by definition. The problem, activity or person is difficult to comprehend and/or deal with. It is probably more accurate to describe that co-worker or adversary that you must handle as complicated. If you truly understood the person, then it would probably not be considered complicated. A personal situation can indeed be complicated and a complicated person is likely the reason why.
Perhaps something can be complex as well as complicated. Not everyone is going to have a perfect understanding of a situation or system and this leads to people perceiving something as complicated. My car is inherently complex. I may think that it is complicated as well and bring it to my mechanic for even an oil change.