How does Delta-Sigma Modulation convert a 1-bit signal to higher resolution signal?
The output of the low-pass filter consists, in principle, of real numbers. This is because, unlike the input values, the filter coefficients are not quantized to one bit and several of them are added to yield one output sample, as well.

Of course, there is no such thing as real numbers in a finite computer; but you may compute the filter output to any desired precision, such as 24-bit.