Many of the cheaper Android phones have older versions of Android installed. Do you upgrade the Android version if you have an older version, or prefer to purchase a new smartphone, when the latest Android version changes?
When i bought my phone it was an Android 10 but over time they sent me and update, after updating i discovered my phone was now at android 11.On my mobile I have android 10, and I have the option to upgrade the version but I don't do it since I can use all the apps I want to use with that version, so until I don't need to upgrade it, I won't do it
I guess you accidentally pressed the update button, it's normal, it's also happened to me with other mobiles.When i bought my phone it was an Android 10 but over time they sent me and update, after updating i discovered my phone was now at android 11.