獣耳会社ｗａｎ on Nostr: It's plagiarism if a part of the work consists of specific material – expression ...

It's plagiarism if a part of the work consists of specific material – expression – that was appropriated and passed off in such a way that a reasonable audience would take it to have been the plagiarist's own work. No passing off, no plagiarism.

If party A appropriates the specific expression embodied in party B's creative work – in this example, some text – and passes it off as their own by not clearly and unambiguously citing it at the time and place the material is used – in this example, during the reading-out of the plagiarized text that accompanies original visuals – then that's plagiarism. (If the plagiarized work is still under copyright protection and was not licensed or otherwise cleared for use, it is also an actionable infringement of the copyright holder's right of reproduction, as well as the original author's moral rights. Adjust for fair use, in jurisdictions where that defense applies, but plagiarism usually acts as a huge strike against it.)

A material's user is usually held to have responsibility for making it clear that the material in question wasn't created by them, which is why various mediums, video included, have well-understood methods for enclosing or otherwise separating quoted/cited material from the rest of a given work.