What is the difference between domain expiry and SSL expiry?

Domain expiry and SSL expiry are completely separate countdowns managed by different systems. Domain registration is the right to use a name (e.g. acme.com), renewed annually or multi-year through a registrar like GoDaddy or Namecheap. SSL (TLS) certificates are cryptographic credentials installed on a web server that prove the site is who it claims to be and encrypt the connection; they're issued by certificate authorities like Let's Encrypt, DigiCert or Sectigo and have their own renewal cycle (currently a maximum of 398 days for commercially issued certs, trending shorter).

You can have a valid domain with an expired SSL certificate (site works but browsers show a security warning), or a valid SSL certificate on a domain that's about to expire (SSL renewal would be pointless). They need to be tracked separately, renewed through different providers, and often owned by different people in an organisation -- which is exactly why both lapse.