Question #1 - Do they have springs in them? - Not anymore and not for a while. 1-sided mattresses have a firm, flat platform as a boxspring. They’re actually called foundations.
Question #2 - Should you replace your boxsprings from your current mattress? - This is always a yes if your are buying a different brand of mattress. Boxsprings/foundations do not last forever and if your mattress is in rough shape, replace the whole set.
Question #3 - Do I even need a boxspring? - Depends on your bed. Platform beds with 10 slats do not require a boxspring. Beds with only 3-5 slats *DO* require a boxspring. You can determine how many slats are under your bed by lifting up your mattress.
Pro Tip!! Measure your rail height if you have a bed(headboard/footboard) to determine what height boxspring you need. 5“ or 9“. Guys!! Impress your lady friends or your significant other by being aware that the boxspring height can help make the room look sharper!