Hi. What's not normal there? The usage on YouTube or the tethering?
Maybe the high usage on YouTube is because of quality of the videos(YouTube may default the streams to a higher quality especially if you are on high speed connection such as 3G or 4G LTE).
For tethering, its a bit hard to tell. What OS do you use? Windows 10 is data intensive OS. You may need to change your hotspot connection to a metered one.