YouTube currently requires that content creators let viewers know "when they use AI to meaningfully alter or generate photorealistic content."
The policy draws some strange boundaries. It applies to "AI-generated music" (not photorealistic) but not to "riding a unicorn through a fantastical world" (this could be photorealistic, though it is not plausible). YouTube then summarizes the policy in a different way: "Realistic AI content and meaningful changes require disclosure, while non-realistic or minor edits don’t."
But AI uses that require no disclosure can include everything from "idea generation" up through "production assistance, like using generative AI tools to create or improve a video outline, script, thumbnail, title, or infographic." Creators are free to clone their own voices for voiceovers. They can also use "AI-generated or altered animation of a missile in a fully animated video."