complex button animation

I have a button animation I’ve been trying to figure out. I did my very best, but I can’t work out how it works. I’d like to send you a link to the website where I saw it. Would you be able to explain it to me?

Here’s the link: https://www.mutabor.de/de/...
Go to the third section on the page (scroll down to Elevate your strategy). On the right, at the bottom, there’s a button called for example Brenntag CASE. When you hover over it, it instantly changes — the button image switches to an icon with an arrow, and the style changes from an outlined button to this icon version.

Thanks a lot,
Geof