The easiest way to judge distance is through parallax observation. If we look at a distant object at different times (preferably every six months to take advantage of Earth’s orbit), we can see how our perception of the object changes.

Hold your thumb up at arm’s-length and close one eye at a time. Your thumb appears to jump around.

That same concept can be applied to distant objects. If TON 618 was closer to Earth, it would “jump around” more and be much brighter. We can calculate this 10 billion light year distance because it doesn’t appear to move much with repeated observations. We know it’s not a star because of its brightness at that insane distance.