Actually BITS is the old thing by now (it's been present in Windows since Windows XP), which only tries to download things in the background by using spare network bandwidth. I've never actively tested how well this "spare bandwidth detection" actually works, but on the other hand I've also never negatively noticed it, so I guess it might actually be doing its job quite well.
On the other hand there is in fact a newer (relatively speaking, given that it's a few years old by now, too) thingy specifically for Windows Update which also tries to use P2P distribution – that one is called DOS. Officially that stands for "Delivery Optimization Service", though personally I rather prefer "Denial of Service service", because when I first encountered I immediately noticed it in a negative way:
Even without any P2P features enabled, it spammed literally dozens of TCP connections to download stuff, and would thereby effectively monopolise the whole bandwidth of my internet connection. Until I figured out what was happening and disabled it (you can disable it and switch back to using BITS for downloading updates), every time my new (at the time) laptop was downloading updates, it would dramatically slow down Internet speeds for everything else in my whole home. Maybe by now that kind of issues have been fixed, but I've never tried it myself because BITS is, as I said, working perfectly fine and unobtrusively.
I have a few Windows machines at home and BITS is so good that I’ve never really noticed it in action.
By comparison, macOS App Store downloads and updates can hog bandwidth horribly. It was just my wife and I on our at the time 25mb/s line, and Netflix dropped to like 240p and was still struggling while I was doing an XCode update.
I'm sorry, not a network guy. But I was thinking of this[0], which says you can update your network through peers. And maybe it isn't bittorrent, but it looks P2P and I'm not sure that's meaningfully different.
The key difference for Windows' Delivery Optimization and traditional P2P is that it's only enabled by default on the Enterprise, Pro and Education SKUs, and then only enabled for connections on the same local network.
And adding to the fun, the one thing I wish it would cache, namely Xbox Game Pass downloads, you can't force it to cache or share on the LAN.
