How does TLS with PQC use key encapsulation?
As there don't seem to be any PQC alternatives for Diffie-Hellman (DH / ECDH), DH must have been replaced by key encapsulation using an ephemeral key pair.

I don't believe that is correct; a postquantum Key Encapsulation Method (KEM) would appear to be the natural replacement for DH/ECDH within TLS.  In the KEM, one side (the client) produces a KEM public key, the other side (the server) produces a response, and they both generate a 'shared secret'; this is what TLS 1.3 needs.
Now, the server can't have a static key (because, unlike DH, the server response depends on the client public key).  On the other hand, TLS 1.3 currently doesn't assume that.
All of the NIST public encryption candidates can work like this; except for the ones that make it problematic by having huge key sizes, they all can fit within the current TLS 1.3 architecture.