Over here and I presume Spain is probably the same, you get a set hourly rate for a “guaranteed” 38 hour week, if you work less than the 38 you will still get the wage, anything over that 38 is on an overtime rate on a graded system, first ■■ hours 25% next ■■ hours 50% , then obviously higher set rates for w/ends and Bk/hols.
Not sure about Spain but here in France you get allowances that are “TAX FREE” : breakfast, lunch, evening meals, night allowance and others.
Wages can be low in Europe, but if you stay below a certain threshold on the taxable income, you wont get taxed, money is made up on the tax-free allowances.
Here the tax year is Jan - Dec and last year I earned for 9 months, salary around € 18,000, brought up to € 34,000 with my allowances.
Before you knock it, find out what extras/add-on’s are added.