This passage is culled from Science Direct, but I don't have the link.
Bone resorption and formation are coupled at specific moving locations. Estrogen limits the formation of those coupled units and balances the cells osteoblasts ( that form bone) and osteoclasts ( that degrade bone). Without estrogen the osteoclasts have longer existance. Estrogen increases osteoblast formation, differentiation, proliferation, and function.
Estrogen also controls (through the kidneys) how we absorb calcium. And how it is used by the body once it is absorbed.