Learn Russian: В молоке есть кальций, к тому же оно улучшает пищеварение. - There is calcium in milk, besides it improves digestion.

It translates to besides or furthermore. Literally it combines the preposition к (to), the dative form of то (that), and the emphatic particle же. While кроме того is a neutral way to add information, к тому же is slightly more conversational and specifically adds a reinforcing point, as if to say not only is the first thing true, but here is a bonus fact.

Could we just say В молоке кальций without the word есть?

Yes, dropping есть is grammatically fine. However, keeping есть emphasizes the actual presence or existence of the mineral. It highlights that milk actively contains calcium, making it sound like a solid nutritional claim rather than just a casual description.

Why do we use the pronoun оно in the second clause instead of это?

The pronoun оно specifically refers back to the neuter noun молоко. If you used это, it would refer to the entire previous situation (as if the fact that milk has calcium is what improves digestion). Using оно clarifies that it is the milk itself doing the improving.

I don't recognize the word пищеварение. How is it formed?