The question is in the title: In both cases I use R² and Chi-Square to test if a fit/model is good enough. Up to now I only know that R² is used for models (?) and Chi-Square for Fits/functions (?). Is this true? And how exactly do they differ? Thank you!
Found this after a quick google: "R^2 is used to quantify the amount of variability in the data that is explained by your model. It's useful for comparing the fits of different models.
The Chi-square goodness of fit test is used to test if your data follows a particular distribution. It's more useful for testing model assumptions rather than comparing models."
sounds like Chi-square is more useful if you have a function you are trying to test (or a model you are trying to fit to your data) as opposed to the R^2 which tells you how much variability there is in your data, and therefore how much the best model fits.