Port forwarding and how do they work?

Question

ADSL

In general, such a question is of interest, I myself am not particularly interested in this topic, but I would like to know why it happens that some ports give out more speed than expected, this is of course an error on the part of the provider, but still, for example, there was such a case that according to my tariff 128kbps but there was such a UDP port, if you redirected it via VPN, it gave out 6-8Mbps and for about 6 months I just redirected to 8Mbps, but in the end the provider passed through and turned it on, so to speak, the faucet is not an isolated case. also a lot of ports that give out more speed than expected and in general I want to know why this is happening? how can i find those ports? I just stumbled upon them by accident, for example, some application gave out more speed and just found out on which port it works and did port forwarding.
Oh yes, I have the Internet through ADSL, well, if suddenly someone is interested