Most offer GPX routes, although the primary navigation method is usually a routesheet that is sent out in advance of the ride. Personally, I navigate off the routesheet using my GPS as a backup (no reason other than that I find I get a better idea of where I am and where I'm headed).
Field sizes can vary - I've done an audax with a hundred entrants, whereas some of the longer ones get a much smaller field.
Unlike a sportive, you'll find participants of all sizes, speeds and bike types. Some will go around relatively quickly, others will be more like the tortoise that pace themselves and get in just within the time cut off - there's no competitiveness, you're only really "racing" against yourself.