In G1: the size of the cell is increasing because its organelles are being replicated and so are the components of the cytoplasm.
In the S phase: the DNA is being replicated, so the chromosomes are splitting into two sister chromatids .
In the G2: Protein synthesis is taking place at this point and the centriole are replicated. The cell is also making a bunch of protein that would be needed during the mitosis phase;
for example the spindle fibres.
I’m not sure if I’ve answered you question properly but that is as much as I know.