I know that this issues has been discussed, and I indeed find information on this forum and try the suggestions out.
It is about the projection of the USA map shapefile in QGIS. I downloaded the shapefile from https://www.census.gov/geo/maps-data/data/cbf/cbf_state.html, and the map stretched out (squished). It looks wider than it should be.
So I tried different projections, including EPSG:102003 USA_Contiguous_Albers_Equal_Area_Conic, EPSG:102004 USA_Contiguous_Lambert_Conformal_Conic and EPSG:102005 USA_Contiguous_Equidistant_Conic, but the map still looks very squished.
I re-wrote the projection file with .txt extension and opened and read it. It showed
Is it possible I still miss some important steps?