Still fuzzy on that one.

Was it a portal like in the original Avengers but Thanos calls them in present time?

or is the lamer "Black Maw figured out how time travel worked and brought the entire army there using Pym particles" theory by the directors work? https://www.looper.com/151852/the-big-thanos-plot-hole-in-endgame-finally-explained/

Makes 0 sense, but whatever - the righteous punched a little harder and won!!