To understand what HTTP/3 involves, you first need to understand the functions of QUIC, UDP, and HTTP/2. HTTP/3 is basically an amalgam of these components. The name HTTP over QUIC already indicates that the data transfer takes place over UDP instead of over TCP.
HTTP/2 uses TCP which is the most common transmission protocol on the internet. TCP processes connections via multi-level handshakes and transmits data packets chronologically. TCP does not resume transmission until a packet has been successfully transmitted. The transmission is secured via Acks, meaning order and delivery confirmations and test numbers. Data transmitted via TCP contains a header with parameters that help sender processes to connect with the recipient's peer processes.
TCP is very reliable in terms of complete data transmission, but is associated with data congestion and loading times, since all transmissions stop until a lost data packet has been successfully transmitted. With HTTP/2, the internet protocol family is reaching its limits, as data transmission cannot be accelerated without new protocols.
Google, therefore, proactively developed its own transfer protocol QUIC. QUIC circumvents TCP load congestion by using datagram-based and connectionless UDP transmission. UDP works like TCP on the transport layer, but foregoes receiver-sender confirmations. Other streams do not have to wait for the previous one to transmit. Round trips between client and server are significantly shortened. The IETF recognized the advantages of the new protocol and introduced it in 2018 as the HTTP/2 successor version HTTP over QUIC.
In principle, the HTTP transport protocol remains the same. It also consists of a header and body, and uses verbs, cookies, and caching. The difference is in the type of data transmission and the presence of integrated encryption.