Clarification on solar / view angles

Hi all, I am new to the world of EO and remote sensing and would appreciate some clarification regarding the angles given in the S2 L1C metadata.
I have parsed through the MTD_TL.xml to get the mean sun and viewing zenith and azimuth angles on my scene. These angles will be input into MODTRAN line of sight editor for atmospheric simulation.

Are the sun angles defined at the ground or at the satellite?

In terms of MODTRAN’s LOS geometry, could you please give clarity about the OBSZEN angle?

Many thanks