I remeber the default setting of the refresh rate is 72Hz and 90Hz is just an option. But after I set the refresh rate to 90 I don't know if 90Hz become the default refresh rate. How can I ensure it?
To ensure it's working at 90Hz, you can make a very simple demo and see if fpscan reach 90. If yes, then it means that refresh rate of the device is 90Hz.
try to open this site and check https://www.testufo.com/
The screen tearing occurs when the refresh rate of your monitor and the frame rate of your GPU are not synchronised. It is distinguished by a horizontal split in one or more areas of the image. If you want to know more about scree ntearing then go through this article to get information on screen tearing and How to Fix Screen Tearing Without Using Vsync.