throttling and congestion both drop packets in a TCP stream, which signals the sender to slow down it's sending rate. congestion means that the packets are lost because they can't be pushed through a networking device (because the output port is busy and there isn't enough buffer, for example if the peering link between an ISP and a busy content provider is at maximum capacity); throttling means there is enough buffer, but the networking device is programmed with a policy to drop packets from certain streams (for example, those with an IP address originating from a content provider)

A VPN circumvents congestion or throttling not by virtue of being encrypted but by having a different source IP or traversing a different route & different network ports/links from other packets going to the high bandwidth use content provider(s)

If at the peering point my ISP has two 1000mbps connections to Youtube and limits them both to 50% speed, that's "throttling". If they set one to full speed and unplug the other, that's "congestion".

From a customer's perspective, the symptoms - slow Youtube videos - are indistinguishable.

Data transmitted using a VPN goes through a different route that might not be congested.

The same amount of data, but not by the same route.