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Feds in Catalonia, Spain think everyone using a Google Pixel must be a drug dealer
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
They're not in memory until the first unlock, that's why there's the AFU vs BFU distinction for cellebrite unlocking devices incl iPhones.
But as the other person said, they can seize your phone and refuse entry. If you need to travel to the USA annually and you don't want them to see your shit, you may want to have a decoy phone that's not logged into your real accounts or have many photos on it. Just enough to make it believable it's your real phone, but not enough to help them forge anything on you.
I am a non-resident US citizen so I believe it would be more difficult for them to search and hold me without trial or legal representation. But these days anything is possible.
By luck of birth I'm pale as a ghost, so as long as they don't unlock my phone and find out what I really think, I should be good. Then I can get back out of the shithole of a country Trump has created as soon as I'm done there.