There are several websites that have free, public domain images, music, and video clips for creators to use in their videos.

Here are a few websites I've used in the past. Search the web for "free public domain [type of content]" to find more.

Free Public Domain pictures: Pixabay; Pexels; Public Domain Pictures

Free Public Domain videos: Pixabay; Pexels; Videvo; Pond5 (historic); Beachfront B-Roll

Free Public Domain music: Bensound (must credit); Freesound (sound effects); FreePD; Incompetech (must credit)

Before using any content in your videos, check the license to ensure it's in the public domain or has another creative commons license (e.g. some content has attribution licensing, which means when posting the video, you need to cite the person whose content you are using).

Some sites have a license or FAQ section that explains the copyright of its content. Other sites leave it up to the content creators to specify licensing when they post the content.