So everyone is saying "to tackle diff. and int. calc you have to master algebra first".
Now, I'm self-studying and I have no idea what "mastering algebra" means in that sense. Functions? Quadratics? Polynomials? Factorization? Complex Numbers? Logarithms? Expo. growth? Advanced functions? All of the above?
What are your opinions for pre-requisite level algebraic knowledge?
Calculus, at its most basic level, requires only a few concepts, most importantly functions. You should also know high-school algebra, things like $(a+b)^2=a^2+2ab+b^2$ and similar should be engraved in your memory.
Exponentials and logarithms are one of the fundamental functions of basic calculus, it is thus very important to grasp these concepts.
Complex numbers, although very elegant, and I personally recommend learning them, are not needed for basic calculus. More elaborate topics make plenty of uses of complex numbers, but it is not so hard to learn the most important facts.
If you want some recommendations on where to start learning, the calculus video series on YóuTube by 3Blue1Brown is a very nice intro, and if you prefer a book, The Calculus Lifesaver by Princeton University Press is also very good.