Android 5.1.1 (Lollipop) is a legacy operating system released in 2015. While it is no longer officially supported with security updates, you can still download and install compatible versions of the to maintain access to apps and services . Latest Compatible Version for Android 5.1.1
For Android 5.1.1 (Lollipop), the latest compatible version of the Google Play Store is typically Android 5
Quality of presentation and trust signals Android 5
Not unless you keep auto-update on. The Play Store runs only when opened. However, Google Play Services background processes may increase – restrict background data if needed. Android 5