Races are mostly arbitrary groupings, based on skin color more than anything else.

Ethnicity, however, is less arbitrary, but still contains some arbitrary factors. It's usually going to be based in culture or national origin, depending on who's using the term.

So, Swedes do have their own ethnicity, though you'd find arguments exactly what ethnicity they'd fall into, but it would likely be different than Brits.

Ojibwa people are very different culturally from, say, Cherokee people. There's even a good degree of common features that vary. But some people will still try to lump them together as "native American", even though that term is almost as useless as "white" or "black".

Truth is, we're all mixed to some degree. Except maybe the sentinel island peoples, or other isolated groups. Even then, it isn't like they didn't get to wherever they are without traveling, so they mixed with something along the way, even if you have to go as far back as when Neanderthals and what gets called modern humans were still fucking.

That's part of what makes ethnic groupings partially arbitrary. It's unusual for no movement between groups to occur, even across pretty damn brutal landscape barriers. Big rivers, mountains, they aren't totally impassable. Even deserts can't keep humans from fucking each other in small numbers as they travel.

However, you can usually go with nationality and ethnicity being linked, though there's so many exceptions that it's absurd to do so. Just look at Nigeria and try to sort out the various groupings there and not notice there's barely an overall national connection between them. And that not everyone in those groupings are even all in Nigeria to begin with.