Question having problem with linked accounts...

Jan 12, 2021
So before this, my brother and I were sharing the same computer and I created a microsoft account for it. Then, after some time, I finally got my own computer so that I can use it by myself. Doing it, I did not realise and accidentally used the same microsoft account that my brother was using. As I started the computer, all the files that were in my brother's computer were in mine. I tried to unlink it and thought it was all fine. However, I came to notice that when I downloaded and installed things, it would also show up in my brother's computer as well. This even includes files like notepads too. How do I completely unlink my brother's account and mine?

Somewhat relevant to the above question, I am thinking of just system restoring my computer but for some reason it does not work. It says system restore failed. Any help on this? Thanks in advance.