This is a common question in this forum.

Maybe you got some new backlinks with low DA or maybe you've lost some high DA backlinks. You can start looking into your linking profile in the Moz Pro report. More info in this old thread: https://moz.com/community/q/why-my-domain-authority-da-is-decreased-from-21-to-19

The redirects are intended to keep link juice when migrating to a new domain. Actually they just help the existing users find the new domain. If there are redirects to your domain from unrelated websites the Google bot should be smart enough to ignore them. That's why it's better to have backlinks from unrelated sources than redirects.

Most youtubers don't mind their videos being shared. The more they are shared the more views and likes they will get. Just make sure to credit the author.

First of all, Google doesn't use Moz spam score: https://support.google.com/webmasters/thread/2440327?hl=en

So, if your Moz spam score is high because of "spammy" backlinks it doesn't mean that Google considers your site as spammy. You can check the spam score for every linking domain in the Link Explorer. If there are too "spammy" domains take the necessary actions (disavow them in GSC as the last resort).

You need to check the HTML code of the link. If it doesn't explicitly say nofollow in the rel tag then it is a dofollow link.

For example, this a nofollow link:

<a <strong="">rel="nofollow" href="https://www.mywebsite.com">Visit my website!</a>

If we remove the nofollow property the link becomes dofollow:

Visit my website!

Apparently Google doesn't care: https://support.google.com/webmasters/thread/21223241?hl=en

From the webmaster point of view it seems like more work to configure each individual property and review every report separately.