Pedantic moment but it’s gonna help explain the next part: all mammal babies can digest lactose, it’s just most adult mammals stop making the enzyme eventually (you still can’t interchange most milks though because the specific balance of water, fat, and electrolytes vary and the inability to adjust to such variants is why mammal babies even need milk).

I remember reading that this one small difference traceable to certain parts of the world but not others (lactose intolerance is more common in some ethnicities than others) was initially exciting because it would be the most advantageous single gene mutation in recent human history but it turned out that even that one small change was still several genes creating various degrees of digestibility under various circumstances and that even more than that it also relies on your gut bacteria. Lactobacillus (literally named these long bois like milk!) is both endemic to the human gut biome and to a lot of stuff we eat like yogurts, and can help you digest or at least not be as bothered by having a bunch of indigestible sugar in your gut.

Sugar molecules in particular are very small and can apply a lot of osmotic pressure at the same volume compared to other things, so they pull a LOT of water into your stool, preventing your colon from compressing and collecting that last bit of water before you poop like it’s normally supposed to. Completely indigestible sugars like xylitol make fabulous osmotic laxatives.