Local "sanctuary" policies don't prevent federal Immigrations and Customs Enforcement agents from acting inside the states, counties or cities that have adopted them.
Under the policies, some jurisdictions have directed their law-enforcement authorities to limit cooperation with federal immigration enforcement. The polices vary in extent and details, but have become more widespread as the Trump administration has sought to tighten U.S. immigration, asylum and refugee rules.
Immigrants not legally authorized to be in the U.S. are under federal law "removable aliens" and subject to deportation. In jurisdictions that do cooperate, local authorities may alert ICE to the detention for other reasons of someone also in violation of immigration law.