So even if you enable 90Hz option from the settings most of the apps will be displayed at 60fps, not 90fps.
What this option does is force all apps to be displayed at 90fps
I tested that with PUBG Mobile "Global" after editing the game files
and now the game is running at 90FPS
To force 90Hz you need a rooted phone and terminal emulator app
just type the following in the terminal:
su -c service call SurfaceFlinger 1035 i32 0