This is the annoying thing about copyright. Is it always required to get explicit permission before doing anything with a copyrighted work?

Not really. You can do whatever you want, and if it violates copyright the copyright holder needs to say that you violated their copyright.

Then if you have an affirmative defense then you can claim that, which you could argue in court.

But really none of that will happen because YouTube's ContentID mostly works with big content creators and nearly all small TikTok creators probably won't bother.