The definition of sanctuary cities per the conservative groups that invented the term are cities that require actual legal process to detain someone, ie requires warrants and the other basic legal process requirements. IE following the Constitution.
My home city is considered a sanctuary city because it requires warrants, just like for anyone else in the US, because the Constitution applies to anyone standing on US soil. It votes like 80% republican. You have literally never heard it referred to as one of the sanctuary cities, but it's on Brietbart and other conservative groups lists as one, and has been there for the past 10 years. It's projected to pass a million people in population in 2022.
What anti-sanctuary-city people want is mass roundups and deportation without due process.