Portus means port in Latin. It is the origin for the name of the city of Oporto, also.
Cala is the Latin name of the Celtic goddess of winter[1], also known as Cailleach Bhéara in Irish or Beira in Scotland. It is also the origin for the name of the region known as Galicia and the Gaia in Vila Nova de Gaia.
Different versions of Galicia exist in most of Europe, from Cornwall to Galatia in Turkey (named after Thracian Gauls).
Southern Poland, some parts of Czechia and (I think) parts of Ukraine were also inhabited by Celts, hence the name of the once Austro-Hungarian province.
In Portuguese it was usually called Cidade do Porto (city of the port), which was shortened to be just Porto.
Nothing modern uses this name anymore, though. I always see "Porto", even in American English.
But, it's a recent enough fix that you can still find signs of it. For example, you can book a flight to "Porto (OPO)". Just like you can book a flight to "Beijing (PEK)".