I'll try to explain.
All 4 are the same basic body with different appearance and price ranges.
Biscayne is the lowest priced.I think it started in 58
Then BelAir.Started in 1953?There were originally 3 BelAir models.the 1500,2100 and 2400(better known as the 150,210 and BelAir)
Then Impala.First year was 1958.
The Caprice was added to the line in 65 as a luxury option on the Impala.
In 1958,Biscayne and Belair became the economy models with Impala coming in as the "Sport" model and top of the line.
All could be had with any engine/transmission offered.With some exceptions.
Each model came with a specific trim and upholstery.
Some models like the Biscayne and BelAir could be had in a 2 door sedan,4 door sedan,wagon etc.
But,the S.S.was Impala only.So was the 2 door hardtop.
Impala and the Caprice were the only 4 door hardtops.
Then,there were the odd option packages you could get if you had the money!
I/6 S.S. Impala's.Bicaynes with 396/4-speeds.Caprices(like mine)with factory installed level air.
65 was a banner sales year for Chevrolet!