With PKI you're trusting a certificate chain up to a CA you already trust, by way of your OS or browser vendor.

A domain can layer on HSTS to that, which directs clients to additionally refuse to trust a new cert for a domain until the one you currently trust has expired.

That’s not what HSTS does. It asks the client to remember that you want to only use TLS for that domain and refuse to use unencrypted HTTP in the future.