Why does Safari add a weird extension to YouTube links?
The Youtube webapp is adding that to the link (or URL, https://en.wikipedia.org/wiki/URL).
The characters after the question mark in the link is the query string (https://en.wikipedia.org/wiki/Query_string). It's one of the ways data can be passed to the server. The &quot;v&quot; parameter specifies which video to show. The ampersand (&quot;&amp;&quot;) separates parameters.
I can't find any reference as to what the &quot;frags&quot; parameter does. &quot;%2C&quot; is a comma (&quot;,&quot;) that's been percent-encoded (https://en.wikipedia.org/wiki/Percent-encoding), so that part is setting the &quot;frags&quot; parameter to &quot;pl,wn&quot;.
It's definitely not a problem. Everything is fine.