Hi! My interpretation of "to come across" is to find something, perhaps unexpectedly or when you weren't looking for it. For example, yesterday I came across this interesting article about learning a new language. Or: Mom, when you're at the store later, if you come across a small ladder, can you buy it for me? For me, personally, I would use "to come across" or "to run across" but you could use them in the same way. I'd say "to come across" is more commonly used (but that's just me). Hope that helps!!

There is no difference. They are interchangeable.