please answer my question 
Can I freely use parts of freecodecamp's tutorial code in the content of my own general manual book?
You should check the code’s licence in Github. For MIT or Apache is a YES but you need to give credits. Other licences will be more complex (if you are unfamiliar.)
where are you searching exactly ?
What you have to check is the source of the code that you want to use. If it is FCC’s code check that github repo, if it’s another repo you search on that repo.
If it doesn’t have a licence you can ask the author otherwise I would normally not use it but if it’s trivial code I doubt anyone would complain.
FCC’s code is BSD licenced, and I don’t remember it off the top of my head but I’d assume that you have to give them attribution and the code must inherit their licence and be open source. You may ask ChatGPT or read about this licence, for example.