I've ridden the past couple of years with a quilled stem and as the belly has shrunk I've adjusted the bar location (height & reach). Now I am interested in buying a new frame that comes with a threadless steerer and realize I don't know how they work, setup wise.

How do you determine at what height to set the stem? Is it based on my present bar height of a different bike?

Do you cut the steerer tube a little too long and ride for a while giving yourself the opportunity to adjust the stem location up and down?

Once you cut the steerer tube there is no adjusting the stem higher, right?

I am assuming there are many different lengths and angles of stems so that over time one can adjust their bar position?

If there is steerer tube protruding above the headset to the stem, is the steerer tube covered with any thing? Or does it come painted?

Thanks y'all.

Yes, there are a variety of stem lengths and angles to choose from, and adjustable angle stems are available to tune in your preference, then install a rigid stem. The steerer is covered by aluminum spacers that are removed once the steerer is cut.

You can try different stem heights without cutting your steerer; you simply put spacers above your stem as well as below. You can then vary the height by shuffling the spacers and stem around until you find a height which suits you. Keep in mind that the combined height of the stem plus spacers will need to be such that the total height is about 3mm higher than the steerer so that the top cap, which is used to set the headset adjustment, contacts the stem (or spacers) rather than the steerer. If the cap touches the steerer you will not be able to properly make the adjustment. Once you are VERY sure you have the height you want you can then cut the steerer if you do not like having spacers above the steerer.

Sheldon Brown gives an alternative method which does not require spacers and can be used to find your preferred stem height here: https://sheldonbrown.com/handsup.html#threadless

I suggest, the longer steerer option, and ride it a while..

though the ride the bike as is, and if you feel it is too low ,

And the factory assembly already cut the steerer.

then you seek out a 'stem riser' ..

I got a particular one that I could use on my 2nd hand trekking bike build,

the whole stem riser is internal, , others clamp on external,
it clamps on the fork the original stem, fits on top of it..

Why not use both?

There's some adjustability since most cut the fork with about 1" of spacers, which can be transferred from below to above the stem. I suggest starting out with an overly long fork, and spacers above the stem until you dial in the height, then you can trim the fork leaving 5mm of space above the stem. I don't like to use more than 1" or so of spacers (though many people use far more than that) and recommend a higher angle stem if more height is needed. (angled stems can be flipped to offer a greater range of adjustment).

Here's a stem angle and height and bar position calculator which will help in choosing the right angle stem for your needs. Work form your existing position relative to the top of the headtube (corrected for any change in headtube length).

To answer your last question, there's no need to paint the steerer tube, since it's never exposed. There has to be a continuous stack of stem and spacers from the top cap to the headset, otherwise the system cannot work.