Let’s be realistic. Upgrading your will not turn your old phone into a flagship. You will still face lag in heavy apps like YouTube (which now requires Android 8+ for the main app, but YouTube Vanced legacy works).
I notice you're asking for a compatible with Android 4.4 (KitKat) . Google Play Store Apk Android 4.4 4 --BEST
like APKMirror (run by Android Police, safe). Look for: Let’s be realistic
– then open Play Store and sign in.