Can someone explain it to me in a detailed way? Force of attraction between a hollow spherical shell of uniform mass density and a point mass situated outside is just as if the entire mass of the shell is concentrated at the centre of the shell.

Dear Student,

By integration we can prove that spherically symmetric bodies (shell or solid sphere), can be thought of as if their mass is concentrated at the center of the body, when their interaction is considered with point masses outside.  For detailed integration steps, you may refer to the book “Concepts of Physics by HC Verma Vol 1”.