The algorithm is super easy:

A stream is defined as something that shares the same Source IP, Target IP, Source port, Target port and protocol, so e.g. "TCP 192.168.0.2:45231 -> 80.0.0.1:443". This 5-tuple is guaranteed to be unique at a certain time.

The router counts how many packets each stream had in the last second.

If the bandwith is at its limit and two streams want to send a packet, prioritize the one that had fewer packets in the last second.

This covers all of your bases:

• A huge download will have tons of packages and it will not have trouble if it gets its speed throttled by a few percent. Because it gets most of the slots it gets lowest priority.
• A video call will have medium traffic. It gets priority over the download, because it has less packets, thus it will not lag, but because it doesn't need more traffic, it will not interrupt the download.
• Scrolling Facebook requires quite low traffic. Most of the content is text or pictures, with a rare, often short low-quality video with quite low traffic requirements. It will get priority over the video call, but since the traffic volume is so tiny it will not harm the video call, unless you have a severely slow connection (<50MBit).
• Telemetry has incredibly low traffic requirements. We are talking about a few kilobytes per hour. These will have high priority, but since they have such an incredibly tiny traffic footprint, that won't do anything at all, unless you are on a 56k modem, but if you are you won't be downloading, video calling or scrolling Facebook anyway.

The cool thing is, this algorithm needs extremely little space in RAM and is extremely easy on the CPU too. To store a stream's data you need 13 bytes for the metadata plus 3 bytes for the package counter, netting 16 bytes per stream. That's 64 concurrent streams per Kilobyte of used RAM. A megabyte fits 64k streams. You can implement that algorithm on an original Gameboy and it wouldn't sweat.

Also, remember that this algorithm only looks at the last second or so of data, so it can drop all idle sessions within just one second. 64k concurrent streams within one second is enough for professional-grade office routers.

Source: I actually spent the first 7 years of my carreer at a company that makes routers and switches and developed the software for them. 128MB RAM was what our mid-range models had, and every single model we had had traffic priorisation.

In fact, our network-provider-grade routers and switches had dedicated network metadata RAM, and they were usually limited to 4-16MB.

The only reason your router even has 128MB RAM or more is so that it can display a fancy configuration web page with images.