I mentioned earlier that MenuLinkTreeElement objects wrap individual menu links, but what can we do with these programmatically if you choose to work with this data yourself and not rely on the menu theme hook? Let's cover a few common things you can do.
First of all, the most important thing to do is to access the menu link plugin. You can do so directly, as it is a public property on the MenuLinkTreeElement:
$link = $data->link;
Now, you can work with the $link variable, which is an instance of MenuLinkInterface, and more often than not, an actual MenuLinkDefault instance that extends the MenuLinkBase class.
So, if we inspect that interface, we can see a number of handy methods. The most common of these will be the ...