take a tree and leave it in a normal nursery pots, shape it and work with it
for a number of years and then in due course place them in to Bonsai pots?
If this is so, could you please explain to me why you can not take a seedling
or a cutting that has been propagated and place immediately into a Bonsai pot
once they have taken and are showing new growth?

Once you place a tree in a bonsai pot, to all intents & purposes it stops
growing, especially the trunk. The goal in bonsai is to have a mature appearing
tree in miniature. To achieve that mature look, and a nice thick tapered trunk,
the tree must be grown for a number of years in the ground, if it is hardy, and
if not, in a large nursery pot. Once you have the trunk the way you want it,
then you can put it in a bonsai pot & work on the branches. Also, for most
species, it is too stressful to do a lot of pruning, styling, & wiring, & then
pot them immediately. I can't tell you how many trees I've killed that way.
Once you do the major styling & wiring, you should leave them in the nursery
pot for six months to a year before you put them in a bonsai pot. If nothing
else, bonsai teaches you patience.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)