Whenever I go to download a game on my pc, it will download fine, but I will lose internet for everything else on my network until the download is complete or I stop the download.
At first I thought this might be an issue with my ISP, but this doesn't happen when downloading a game from my Steamdeck.
This. Link instability and packet loss is common when the link is being utilized to its max theoretical capacity, as opposed to your ISP having a soft bandwidth cap.
Cap your download speed in steam settings 20% lower than your max download speed (possibly lower) to give the link layer some breathing room.
If this is the cause, essentially the ISP is buffering a lot of data on their side to make sure it's ready to send to your router ASAP, but that also means that anything else being requested gets queued behind all of that Steam download data, so it takes forever for anything to load although it should eventually load.
You can also try pinging google or something while downloading, you might see the ping response times go from a few ms to seconds.