The same way they made World of Warcraft, I suppose. (That is, until they changed it to be the same as D4) Or the same way From Software made Elden Ring. All they really had to do was stick to the formula they established in the first 3 games, because the map being bigger doesn't have any bearing on the RPG mechanics or anything. There's just more walking now.

It's a lot of work to balance all of the enemies for all of the different types of players, but that's kind of what I expect when I pay $70 for a game with microtransactions