The easiest way to explain it is:
DPL: your club has to apply and be accepted to play in DPL. It is usually the second (B) teams from clubs who are playing in GA. Although there are some clubs that don't have GA and have their top teams playing in DPL. It is a national league that has showcases throughout the country. It is only for Girls teams. likely more travel for league games compared to NPL.
NPL is the highest level available to teams that play in SoCal. Both boys and girls play NPL. It is a team by team league, meaning your clubs top G05 may play in NPL but your G06 could play in Flight 1. no home or away games, all games are at a single location (ie silverlakes, oceanside). For larger clubs that have GA/DPL/ECNL/ECN_RL the teams who play in it are their 3 or even 4th teams. For smaller clubs that don't have GA/DPL/ECNL/ECN_RL this is usually as high as you can go.