"respect to x" kinda means you have to apply the operation on any function of x. So differentiate x with respect to x means you apply differentiation on the function x, hence it is 1. When you integrate with respect to y, you apply integration on the functions of y (e.g 3y, y^2 etc) , so anything that doesn't contain a function of y in the integral cannot be integrated unless you substitute it for a function of y.
A constant in an equation is anything that is just a number and doesn't contain the variable. For example in the equation, x^2 + x + 1 = 0, the 1 is the constant term because it has no x attached to it.