Since MIUI updated to version 11, it restricts setting your default launcher. You can only set “verified launcher” as default. But its easy to bypass that.
Xiaomi created a custom activity for choosing the default launcher. If you did not download the launcher from a “trusted source”, a dialog tells you that you can not select it.
Luckily Xiaomi did not remove the normal activity. You can open it with the following command
am start -n com.android.permissioncontroller/com.android.packageinstaller.role.ui.HomeSettingsActivity --user 0