Next to my daily driver I also have an Xiaomi Redmi 7 running MIUI 11. The preloaded Xiaomi Applications such as the File Manager contain built-in Ads. It is known that Xiaomi has low-margins to hit certain price points while keeping the components relatively high.
Fortunately, there is a fairly easy work-around to disable the ads by revoking the system permissions that allows the MIUI System Ads daemon (msa) to collect data and push ads to you.
The ads first started appearing with MIUI 9 and with version 10 and now 11 available for the newest handsets the place in the settings menu has shifted multiple times. The following steps apply for MIUI 11 as of December 2019.
Go to Settings –> Password and Security
Select Authorization & revocation and toggle the switch of mda to revoke the authorization. A count-down will appear warning you that this operation might break the application which in this case is actually what we want in order to disable the ads.
The toggle should be disabled after the revocation. Enjoy the ad-free experience.