A question regarding copyright

It depends what usage rights the creator defines.  There's several standard types that allow for reuse:

Public Domain

Fair Use

Free for non-commercial use

Free for commercial use

Attribution

Share

Remix

Share Alike

And any combination of the above plus any I forgot.

It's your responsibility to find out what the usage rights are on any asset.

You need to spend some time studying the different types of usage rights.

Most websites that host images and videos have a link somewhere about usage rights.

Sometimes they are somewhere near the individual photo or video such as on Deviant art or Vimeo

Others have a general description somewhere on the site such as government sites and Archives.

You have to find this information out.  Somethimes it's obvious and sometimes it isn't

Sometimes you have to email site administrators and wait for a response.

Sometimes finding out who you need to ask and how to ask them requires a little detective work.

As for Youtube...Be Cautious...unless otherwise noted everything on Youtube falls under a General Youtube License which states you cannot resuse any content without explicit permission from the license holder.

Problem is, Youtube is a human cesspool.  Half of what people claim as their own on there is stolen to begin with.

Even if the uploader says it's free to use you are still rolling the dice.

If it turn out they did't have the right to distribute the material to begin with you can still get nailed.

Ignorance of the law is no excuse.

If you're taking something just to practice your technique and it's never going to leave your computer and

you are never going to distribute in ANY WAY, then go for it. Nobody will ever know.

But if you plan on uploading the finished work, even non-commercially, then you take your chances.

The only sure bet is purchasing templates from reputable sites and even then you have to be aware of the usage rights associated with each individual purchase.  Some templates have restriction limit how you can use the end product as well as how many times you can use it.