Why is it not "У неё есть яблоко"?
It's probably wrong. Report it!
Same question here.
This is actually right, while "У неё яблоко?" sounds really weird.
In spoken Russian, the «есть» part of the expression is often omitted.
What does ˝неё˝ mean exactly?
It is one of two words that could be translated as "her". The other one is "её".