Cognitive overload? From IRC and mailing lists? It makes me doubt whether you’d be able to contribute anything, anyway.

UDP is hardly a transport layer protocol. It basically adds port numbers and checksum to IP. QUIC is usually described as transport layer since it provides flow/congestion control functionality usually ascribed to transport layer.

There is no allocation if you haven’t configured any. Whoever can get their shit stuffed in the pipe first wins. From that point, any bottlenecks either FIFO to/from buffers, or if buffers fill up, just start taildropping. TCP (and other transport layer protocols like QUIC) implementations then have a sliding window algorithm that figures out the optimal amount of data to keep in flight at one time based on RTT and any packet loss caused by taildrops along the way.