The number of connections isn’t relevant. A single connection can cause the same problem with enough traffic. Your bandwidth is not allocated on a per-connection basis.

If you download 2 separate files over HTTP, you'd expect each to get roughly 1/2 of the available bandwidth at the bottleneck.

With 1 HTTP connection downloading a file and 100 BitTorrent connections trying to download a file, all trying to compete, you'll find the HTTP throughput significantly reduced. It's how congestion control algorithms are designed: rough fairness per connection. That's why the first edition of BBR that Google released was unpopular, it stomped on other traffic.