score:19
The likelihood is extremely low unless you have have special circumstances that trigger advanced interest.
In 2018 there were about 33000 phone and laptop searches and there are about 80 million visitors. The statistical likelihood would be around 0.0004 .
**UPDATE based on comments **
Sources: Number of US visitors: https://uk.usembassy.gov/international-visitors-to-u-s-on-the-rise-infographic/
Number of searches: I can't find the original source quickly but here is a Washington Post article that quotes the 2017 numbers at 30,000 https://www.washingtonpost.com/world/national-security/us-customs-agents-are-searching-more-cellphones--including-those-belonging-to-americans/2018/01/05/0a236202-f247-11e7-b3bf-ab90a706e175_story.html
Upvote:3
Yes, they can and will ask you to log into your phone, and search your phone for "obvious stuff" - emails, texts, chats, Facebook, etc. Generally, what they're looking for is evidence of one of the big four:
But if they find something else ... well, gotcha.
They may also plug your phone into an automated process that scans your files for keywords or illegal/obscene files (typically by image or hash comparisons).
Keep in mind that obscenity-wise, something legal in both nations may be illegal to take across the border! (this, page 7, prohibiting perfectly normal mainstream stuff like vanilla hetsex, Fifty Shades of Grey, and contortionists of all things).