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1. not reuse copyrightable content when you can easily avoid it, but instead express your ideas in your own words, and with your own pictures and code;
2. when you do want to reuse content created by someone else, ask their permission if possible;
3. when basing your work on freely licensed content such as SO posts, try to follow the spirit of the license as far as possible, even if you cannot technically follow it to the letter; and
4. in any case, always give credit where it's due, both for (potentially copyrightable) content and for (non-copyrightable) ideas and inspiration.

1. not reuse copyrightable content when you can easily avoid it, but instead express your ideas in your own words, and with your own pictures and code;
2. when you do want to reuse content created by someone else, ask their permission if possible;
3. when basing your work on freely licensed content such as SO posts, try to follow the spirit of the license as far as possible, even if you cannot technically follow it to the letter;
4. in any case, always give credit where it's due, both for (potentially copyrightable) content and for (non-copyrightable) ideas and inspiration; and
5. generally try to be nice to people whose knowledge, creativity and hard work you're building on.

Of course, following this advice won't prevent someone from suing you for copyright violation (although it might in some cases reduce their chances of prevailing, and/or the damages they can successfully claim if they do). But it makes it less likely that they'll want to.

Neither of those licenses is exclusive, so you're certainly allowed to also license your video (to YouTube and/or anyone else) under some other terms. But even if you do so, your original license grant to YouTube still applies. You can't just turn around and say that haha, you were just kidding, it actually doesn't.

TL;DR: Technically, the SO and YouTube licenses are (probably) incompatible, so if you want to use copyrightable elements from SO in a YouTube video, you'll have to get permission from the original author. Certainly it never hurts to ask, even if you might not strictly need it.

Neither of those licenses is exclusive, so you're certainly allowed to also license your video (to YouTube and/or anyone else) under some other terms. (That's what the answer you linked to means by "[stating] a license in your video description".) But even if you do so, your original license grant to YouTube still applies in addition to the other license. You can't just turn around and say that haha, no, you were just kidding, it actually doesn't.

By the way, one of my own answers on Physics Stack Exchange was actually turned into a YouTube video by the "It's Okay To Be Smart" channel a couple of your ago. What they did was exactly this: while they followed the style and structure of the answer quite closely in many respects, they took care to rephrase everything in their own words and to recreate all the demonstrations and diagrams.

Is their video legally a derivative work of my answer? Maybe, marginally — some of their diagrams look very similar to mine, and some of the similarities just might be distinctive and original enough to be copyrightable. But even if I wanted to sue them, this would be a tough argument to make and the outcome would by no means be certain.

I do wish they'd at least included my name and a backlink, though. :/