When congestion happens at a bottleneck in a switched network, bandwidth is (somewhat) evenly shared by default. For instance if you interconnect two switches with a single 1G link and then try to transmit two flows of potentially 1 Gbit/s across those switches, each flow will get 500 Mbit/s pretty exactly.
With most switches, congestion causes frames to be dropped by source port: one server trying to send one flow of 800 Mbit/s and another server trying to send two flows of 500 Mbit/s across the two switches from above will end up the one stream at 500 Mbit/s and the others at 250 Mbit/s each - half of the total bandwidth for each source port. However, this depends on the exact hardware at hand and should be tested before relying on it. The effective throughput also depends heavily on any congestion control that takes place on higher network layers like in TCP.
If you need more control, you can use QoS settings to prefer one kind of flow over the other kinds (or define different priority classes). How this exactly works also depends on the hardware you're using. Some switches support more priority classes than others and some support more scheduling schemes than others.
Jitter also depends extremely on the exact pieces of hardware.