Residential Railing For Interior Steps Questions, Please

This is going to be tough to describe without a picture, which I understand
would be bad form to post with (assuming it's even possible).
I'm not a carpenter or woodworker, so am probably not describing this very well, but let me try.
Moving into a 30 yr old split level house that has the staircase banister "assembly" between the floors made from rough pine. It does not have baluster posts to the step level as a modern railing does; rather it uses fairly closely spaced square post pieces from the top hand rail molding to the level of each step, at the edge, where it is fastened to the side of.
These posts are screwed into the Sides of the step assembly. They are not fastened as the more modern typical baluster posts would be thru their bottom.
There is also an upper loft room, overlooking the living room below, which has a
"safety" railing of the same construction - also fastened by screwing from the side at the bottom of the square (baluster type) posts.
I wish to replace all of this with a better grade of wood, and probably a bit more modern looking, and definitely stronger and more secure for kids leaning against it.
a. My main question is whether one can buy pre-made sections to install,
rather than the one baluster post at a time approach ? If so,from who, please ?
Or, is this approach not practical because all staircases, probably, have a different slope ?
b. Is this screwing it in from the side at the bottom approach used any more,
or are all modern banister installations done by fastening the balusters thru their bottom ? Why ?
c. Are there particularly one or two "major" suppliers on the web to look at that show really pictures ?
Found several, and have learned a bit, but was wondering if there are any that are in particular good to look at,
and possibly deal with ?
d. Before trying to find a carpenter, I would like to learn a bit more as to what might be available, and different approaches to consider. Any thoughts on this would be most appreciated.
BTW: for anyone living in the Boston metro area: any suggestions as to where to see some of these railing possibilities
locally ?
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I've never seen them, but that does not mean they don't exist.

Similar, but they can vary slightly.

Sounds ugly and cheap. My outside deck is like that, but I'd can understand why you'd want to change the interior rails.

Some balusters are more or less set in recesses rather than attached. I don' tknow if any codes exist on that.

Look for a real lumber yard that carries that sort of thing. Not Home Depot, but a real place that carries what you want to see. You can buy al the components you need, various styles, etc. My local lumber dealer was most helpful when I replaced mine.
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