How to specify integration limits in scipy.integrate.trapz or simps?
but at what value does it start x

It doesn't matter for integration. You just specify the values of y and tell scipy how far the xs are apart. Whether they start at -5 or +26 doesn't influence the value of the integral.
I understand that in scipy.integrate.trapz(y, x=None, dx=1.0, axis=-1) or simps, the min and max values of x (if specified) are taken to be the limits of the integral, but what happens when x=None? It has dx to figure out the spacing in the x values but at what value does it start x? 
I tried it with and without x, from which I understand that it starts x values from 0.0 when x is not specified.