I didnae ken how! Thank you!
Considering you know where the "Insert Link" button is,the button on the right,"Insert Image",is how you embed image URLs,including GIFs.
However,when putting GIFs,make sure there's no junk in the URL that isn't needed,and for Tenor GIFs specfically,when looking at the GIF on the Tenor page,open it in a new tab,head to that new tab,then open it in a new tab again,this final result gives you the actual URL needed to ensure trouble free embedding.
Go two more buttons to the right,and you get the "Media" button,it's lost most function outside of some Twitter posting(which is unstable at best,doesn't work at all for the most part)and is the button needed for YouTube vids.
However,like with GIFs,you need to ditch the extra crap,and ensure it's the proper "youtube . com" and not the shortened versions.
This method also allows shorts,since using the proper one turns the short into a proper YouTube video.