So a bit of clarification about plagiarism vs copyright infringement, from someone who has worked in the realm of commercial publishing:
Plagiarism is, ultimately, a moral issue. If you copy someone else's work and pass it off as your own, that is plagiarism. If you get caught doing it, it will reflect badly on you and may result in tangible or intangible consequences (e.g. fines, loss of income, loss of reputation, etc). However, plagiarism is not actually illegal, because it is a moral concept, not a legal one.
Copyright infringement is where all the legal stuff comes into play, because artists & companies that own intellectual property want to protect it from theft or unauthorized use. Some companies (especially those with deep pockets) are more aggressive about this than others, so if you are picking up and incorporating their work in some way (e.g. taking a verbatim passage or using an image) that doesn't involve significant alterations, you are going to want to ask permission and/or be prepared to pay associated fees for use of their work, otherwise, you could get into legal trouble. (Side note: You don't necessarily need to commit plagiarism to run into copyright infringement issues! Remember, plagiarism is about passing the ideas of others off as your own; copyright is about who owns those ideas and under what circumstances they are willing to let others use them.)
That said, almost all art is derivative in some way other, and this is where the legal concept of Fair Use comes into play (at least in the US). Fair Use is a doctrine that promotes freedom of expression and provides some protection to authors and artists who use take inspiration from other work as long as they are using it to produce something new and original. For more info about what constitutes Fair Use: https://www.copyright.gov/fair-use/more-info.html. Please note that this is specific to the US, so depending on what country you live in, and what country's laws govern the publication of the work you want to use, other rules may apply.