If I research topics in a Twitch stream (google) and show other public websites or PDFs, I then violate the respective copyright or can I do it with my own identification, e.g. The name, link and possibly the author of the website, like in a scientific work, for example, as a "quotation" with a reference to the source?
A quotation from a protected third-party work in the sense of UrhG https://www.gesetze-im-internet.de/...g/__2.html is only permitted under very strict conditions. Examples of this can be found in UrhG https://www.gesetze-im-internet.de/.../__51.html and in many texts on the Internet.
Now if you broadcast a Twitch stream claiming that more and more websites were teeming with grammatical errors,
then examples may be quoted, insofar as this is absolutely necessary for your work - but only to the extent necessary.
If something unnecessary becomes visible - even when scrolling through websites and clicking next - then the "public playback" of this content could be viewed as a
https://www.gesetze-im-internet.de/.../__57.html be permissible:
Duplication, distribution and public reproduction of works are permitted if they are insignificant accessories
next to the actual object of reproduction, distribution or
public reproduction are to be viewed.