Hi, how does physics graduate school work in Spain? What are the admissions tests?
Have you checked out the website of universities over there?
The question you have is very broad.
I assume it is roughly the same as in every other country. For example in Madrid you have this http://www.upm.es/internacional/Students/StudiesDegrees/PhD+studies/Admission
One of the first results when I searched google for "University Madrid" was this university and by clicking around a little I already found it.
Separate names with a comma.