The Census Bureau publishes a ZCTA to Census Tract Relationship File, available at https://www.census.gov/geographies/reference-files/2010/geo/relationship-files.html#par_textimage_674173622. Both ZCTAs and census tracts are built from block groups, but census tract boundaries do not align with ZCTA boundaries. This is seen if you turn on both ZCTA and census tract boundaries in the Boundaries & Optional Layers tool.
