HTTP/3 is made for QUIC so it takes full advantage of QUIC's streams, where HTTP/2 had to design its entire stream and multiplexing concept of its own on top of TCP.
HTTP requests done over HTTP/3 use a specific set of streams.
HTTP/3 means setting up QUIC streams and sending over a set of frames to the other end. There's but a small fixed number (actually nine on December 18th, 2018!) of known frames in HTTP/3. The most important ones are probably:
HEADERS, that sends compressed HTTP headers
DATA, sends binary data contents
GOAWAY, please shutdown this connection
The client sends its HTTP request on a client-initiated bidirectional QUIC stream.
A request consists of a single HEADERS frame and might optionally be followed by one or two other frames: a series of DATA frames and possibly a final HEADERS frame for trailers.
After sending a request, a client closes the stream for sending.
The server sends back its HTTP response on the bidirectional stream. A HEADERS frame, a series of DATA frames and possibly a trailing HEADERS frame.
The HEADERS frames contain HTTP headers compressed using the QPACK algorithm. QPACK is similar in style to the HTTP/2 compression called HPACK (RFC 7541), but modified to work with streams delivered out of order.
QPACK itself uses two additional unidirectional QUIC streams between the two end-points. They are used to carry dynamic table information in either direction.