This is all a family history, starting with the 737-100. Next came the -200 with a longer fuselage and bigger engines. Next in line is the 300/400/500. It had the same wing as the -200 but with a new fuselage, new engines and upgrated cocpit. The wing on the 300-500 was never really satisfactory, it was too small for the weights of the newer models, particulary the -400 which is a real sled at all up weight. It is really hard pressed to make FL300 fully loaded.

The NG series have the same basic fuselage as the 300-500 but with upgrated systems, new cocpit and a new wing. Also included in the upgrate is a longer landing gear and new engines.

There is nothing left of the old 737-100/200 anymore. It is a completely new aircraft, but still flying on the old type certificate. Saves a lot of money on the certification program.

Hope this clarifies a bit 😀 😀