US border control - do they really check everyone’s phone?

I’ve been hearing a lot regarding US border control asking to see anyone’s phone for no reason at all, even ones with a return ticket and valid booked accommodation/proof of itinerary. I understand if you refuse to show your phone you will be sent back to where you travelled from but what exactly are they checking on an innocent civilian’s phone who is travelling there for a simple holiday? Return ticket, accommodation and funds sorted etc.

I’ve also read posts of these people getting rejected even with proof they are simply there for a holiday?

And would you recommend bringing exchanged dollars or no?

(Going from UK to LAX airport)