Quick take
A common order is pressure tank → sediment/oxidation treatment → iron/sulfur filtration → softener → polishing → UV → optional point-of-use RO. Testing should determine the exact train.
Pressure equipment stays upstream
Treatment is typically placed after the pressure tank so the well pump and pressure controls operate normally.
Remove fouling contaminants before softening
High iron, manganese and sediment can foul softener resin. Dedicated pretreatment should usually remove those loads first.
UV belongs near the end
UV performs best on clear water, so it should generally follow sediment and chemical treatment.
RO is usually point-of-use
Whole-house RO is expensive and unnecessary for many wells. A kitchen RO stage can handle dissolved drinking-water contaminants after the whole-home treatment train.