Why is X non-random in Linear Regression? What does it mean for X to not be random?

Why is X non-random in Linear Regression? What does it mean for X to not be random?

Proof that the Sum (xi - xbar)xi = Sum (xi - xbar) * (xi - xbar) = Sxx

Proof that the Sum of (xi - xbar) = 0

Deriving the Binomial canonical link function, logit, for Generalized Linear Model (GLM)

GLM Link Function, and Canonical Links

GLM Exponential Family, Example with Poisson distribution finding functions a, b, and c

GLM Exponential Family, Prove that Var(Y) = b''(theta)*a(phi)

GLM Exponential Family, Prove that the mean is the first derivative of b, E(Y) = mu = b'(theta)

GLM Exponential Family, Example with Normal distribution finding functions a, b, and c

Simple linear regression, Variance of a residual, Yi and Yhat independence. Plus or Minus sign?

Deriving Confidence Interval vs Deriving Prediction Interval

Proof for the Probability of a Union of Three Events

Show that A∩(BUC) = (A∩B)U(A∩C)

Using Venn Diagrams, show that if A is a subset in B, then B complement is a subset of A complement

Why is the Variance of the Sample Mean equal to Sigma^2/n ? How to find the Variance of X-bar

Where did 1/(1-exp(x*beta)) come from in Logistic Regression?

How to Derive the Maximum Likelihood Estimators for Logistic Regression

Prove Sum yi(xi - xbar) = Sum (yi - ybar)(xi - xbar)

How to Derive the Score Vector for the Maximum Likelihood Estimators of a Logistic Regression

Set Theory Example: A Complement Intersection B = A' ∩ B = A^c ∩ B

Set Theory Example: Intersection of three Events: A Intersection B Intersection C = A ∩ B ∩ C

Set Theory Example: Union of three Events: A Union B Union C = A U B U C

Set Theory Examples: A Intersection (B Union C) = A(BUC) = A∩(BUC)

Set Notation Examples: Sample Spaces is the real number line: Find Complement, Union, Intersection