Couple of other points:

DL's domestic flights can arrive at either Terminal 3 or 2. You will have to leave the terminal (i.e., secured area) to get to Terminal 1 as all these terminals are stand-alone terminals (though T3 is connected to T2 by a walkway).

If you don't have bags (or if your bags are checked through), your best bet (if you want to avoid the vehicular traffic outside) is to exit the DL terminals via Terminal 2 (even if your flight arrives at T3, follow the signs and walk to T2 - the furthest gate in T3 to T2 will take around 10 minutes to walk). After that, T2 is very, very close to T1 - it is literally a 3-5 minute walk.