There is no blanket answer
A cellphone can hold highly private information. Whether police may search it depends on the legal authority relied on and the circumstances. A lawful arrest does not give unrestricted permission to examine everything on a device.
The Fearon decision
In R. v. Fearon, 2014 SCC 77, the Supreme Court considered limited searches incident to arrest. The framework requires a lawful arrest, a valid purpose connected to that arrest, a search tailored to that purpose and detailed police notes. Warrants and other legal powers can involve different considerations.
Get advice about the actual search
Tell counsel when the phone was taken, what police said and whether documents were provided. Preserve relevant records. A lawyer can review the authority, scope and potential Charter issues; an unlawful search does not automatically mean charges will be dismissed.