Difficulty absorbing Idea of interpolation?

1. Is interpolation a low pass filter?
2. Which filter is used in interpolation?
3. What type of interpolation does a square pulse use in order to reconstruct an analogue signal?

Is interpolation a low pass filter?

Interpolation is a two step process: upsampling followed by low pass filtering.

Which filter is used in interpolation?

The interpolation system uses the lowpass filter H(z) after the expander to attenuate the frequency components of the signal from f_s/2 to Lf_s/2. In the time domain, the effect of H(z) is to replace the inserted zero value samples that the expander introduces with the interpolated values.

What type of interpolation does a square pulse use in order to reconstruct an analogue signal?

The ideal is the Whittaker-Shannon interpolation formula, or sinc interpolation. This is equivalent to convolution of the signal data with a sinc function, the impulse response of an ideal lowpass filter, at half the sample rate.