Thank you for the useful infos. I learned some new things from this. For example I didn't know anything about purchasing licenses before. Since I usually use my own pictures and videos I didn't get to that yet. But I have a question: if I write a post referring to for example a YouTube video can i post the link with source and creator or is this harming the copyright too?
Yep, you cannot use other peoples work without permission unless it is CC0 Licensed. Keep in mind that you're on a platform that requires a commercial use license, whether you get paid or not. Hope this helps.