Solved! My friend's phone is stuck on screen with white background and grey apple logo.

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My friend’s phone is stuck on a screen with a white background with a grey apple logo in the middle, it periodically turns itself on and off but never shows anything but the screen mentioned before. I have tried updating it but it doesn’t have enough space. The phone is an Iphone 6s Plus

 

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Can you backup data to a Mac or PC?

you can get into safe mode:

View: https://www.youtube.com/watch?v=I-qtCUI8lGg

Is it jailbreaked?

After this you could factory reset the phone