MIUI is Xiaomi's variation on Android that is installed on this OEM's myriad mobile devices. It is known for being particularly fully-featured and customized along this company's own lines. Then again, it has typically blanked on some useful user-experience points.
They include an EQ for its MiSound app, even in its most up-to-date form based on Android 10. Judging by the volume of apps that deliver this feature currently available on the Play Store, this feature may be welcome among fans of this OS.
However, some XDA and Xiaomi.eu (a platform for custom MIUI development) contributors claim to have detected code for one in a recent MIUI 11 nightly (v9.12.19). These builds may be pushed to interested users so as to drive evolution in the official version of this build. The EQ in question is relatively basic, having only 7 bands, but the point is it's there in the system, independent of the need to download anything else.
This update also brings further features to MiSound. For example, it adds a screen for increased optimizations for Xiaomi-brand wired headsets (of which there is considerable variety). There is also a menu in which the user can choose pre-set volume and pitch levels designed to suit different age-ranges. To date, these new MiSound options are not enabled by default, not even in new MIUI 11 nightlies.