Physicists have estimated the universe to be flat and infinite, but I can't seem to understand what this actually means. First of all, what do they mean when they say that the universe is infinite, do they mean that space is unbounded? Or do they mean that the space is currently bounded and is expanding forever (so the bounds are getting further apart) ? And if space is unbounded, how does it expand, how can infinity get larger? Finally a flat universe could also be bounded if for example it has a toric shape which has zero curvature, could this be a possible shape of the universe?
What also confuses me is that, since the beginning of time, the universe had a very small size, but how did it become infinitely large in a finite amount of time?