I mean, it’s kinda like the contrast between “cow’s milk” and “milk cow” in English (though we ruin the contrast by typically saying “dairy cow” instead). Fun fact, there’s a lot of times in Japanese where you’ve got two words which consist of the same pair of kanji switched. The most common one you’ll probably come across all the time is 日本 versus 本日, and at your level you should have already come across the 光栄 / 栄光 pair on Wanikani.
The trick to remember (in general for all jukugo, not just in these specific situations) is that the first kanji describes the second kanji, so 牛乳 is cow-milk and 乳牛 is milk-cow.
ら is just a slightly rougher pluralising suffix than たち, not really dialectical. (If there’s gonna be any dialects around here, though, it’ll be Hokkaido-ben.)