Sandra finds her mobile phone very useful.
She uses it all the time
She has been using it all the time.
Which one is correct in the second sentence? Please tell me!
As is often the case with English tenses, both are completely correct and natural, and might even refer to the same objective circumstances. The difference is entirely in how the speaker is choosing to express the temporal relations in what they are saying.
If they say She uses it all the time, they are leaving the time unbounded: her use probably includes times from the past into the future (though if she stopped using her phone right now, that would not make the statement false).
If they say She has been using it all the time they are reporting on a period from a time in the past up to the present. That doesn't say anything at all about whether she will do so in the future: the speaker is choosing to focus only on the past up to the present.
Note the difference between them. Uses: More like a frequent word. See has been using. Has been using: This phrase emphasises that she has been doing this activity since a specific time. I suggest adding the word since and a specific time. Don't rely too much on this answer as I AM NOT TOO SURE. Thank you. Please upvote if you find this helpful.