Follow the path :
Setting - - > additional setting -- > Privacy -- > ad services . You will find “Personalized Ad Recommendation” disable it.
How to disable ad of the specific app.
How to disable ads from Mi File Explorer App:
How to disable ads from Mi Music:
If you are using default web browser from MIUI and want stop receiving ads then do the following:
Why Xiaomi shows ads
This company sell its handset on low-profit margin compare to other smartphone makers. In short, to earn some profit it promotes advertisement with its stock apps like browser, security or file manager. The only good thing about it is that the company has given the option to disable it.