From the guide:
At the U.S. border. Agents may ask travelers to unlock their devices, provide their device passwords, or disclose their social media information. This presents a no-win dilemma. If a traveler complies, then the agents can scrutinize and copy their sensitive digital information. If a traveler declines, then the agents can seize their devices, subject the traveler to additional questioning and detention, and otherwise escalate the encounter.
Border agents cannot deny a U.S. citizen admission to the country. However, if a foreign visitor declines, an agent may deny them entry. If a lawful permanent resident declines, agents may raise complicated questions about their continued status as a resident.
Your response to this dilemma may vary according to your risk assessment. However, all travelers should stay calm and respectful, should not lie to border agents or physically obstruct them, and should plan for this dilemma ahead of time. Try to document or politely ask for the names, badge numbers, and agencies of the government officers you interact with.