Just a thought, but I assume you can upgrade Android 5.1 Lollipop to a newer version in order to benefit from the extra security that newer Android versions feature, partly due to vulnerabilities being patched.
My personal Smartphone (Samsung Galaxy S7 Edge) came with Android 6.0 Marshmallow, and I subsequently upgraded it to Android 7.0 Nougat and finally upgraded it to Android 8.0 Oreo.
AFAIK, this is possible both on low-cost mobile phones and premium mobile phones.
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