Primary Domain is Expired but Website is Still Active

I'm currently working with a client who has their website here on Squarespace. I'm trying to understand what's going on in my client's account.

The domain shows that it expired back in March but the website is still active. I'm trying to understand what's happening here. I see that Squarespace has a built-in domain that never expires. but the site is still viewable through the website's regular URL...not the built-in domain.

Can someone explain what's happening here. Thank you.