So let's say for some function f(x,y,z), I want to find what would give the highest value that would be maximization, right? And if I wanted to find the lowest value that would be minimization?
Yes, that's by definition true.
So if a genetic algorithm is a search algorithm trying to maximize some fitness function would they by definition be maximization algorithms?
Pretty much yes, although I'm not sure a "maximization algorithm" is a well-used term, and only if a genetic algorithm is defined as such, which I don't believe it is strictly.
Generic algorithms can also try to minimize the distance to some goal function value, or minimize the function value, but then again, this can just be rephrased as maximization without loss of generality.
Perhaps more significantly, there isn't a strict need to even have a function - the candidates just need to be comparable. If they have a total order, it's again possible to rephrase it as a maximization problem. If they don't have a total order, it might be a bit more difficult to get candidates objectively better than all the others, although nothing's stopping you from running the GA on this type of data.
In conclusion - trying to maximize a function is the norm (and possibly in line with how you'll mostly see it defined), but don't be surprised if you come across a GA that doesn't do this.