Wiring in a new light fixture and switch to existing switches?
There are no neutrals here.
That's exceedingly important to understand about a switch loop (extremely common, no longer to current codes, but perfectly fine in older codes, so you will find them in every moderately aged house.)
Given your 12V experience you probably think of colors as being fixed things - this is not quite true in household AC wiring.
The &quot;sheathed, 3-wire feeds&quot; are 2 wire cables (the grounds are not counted in residential cables. So a 14/2 or 12/2 cable has a black, a white and a bare  - 3 wires, but it's a /2 cable.) A wire is a single conductor, a cable is multiple wires bundled in an outer sheath.
Modern code would require a /3 cable to the switch location (black, white, red, bare) but you don't have to change the cable to &quot;modern code&quot; unless you are remodeling (changing a switch is not remodeling.) That is &quot;grandfathered&quot; meaning it met code at the time it was installed.
However: adding a cable to an unwired switch IS going to require using a /3 cable from someplace that does have unswitched hot and neutral, or two /2 cables (with white as neutral in both) if the place the power comes from (unswitched hot) is not the place the light (switched hot) will be.
The white wires to the switches are hot. They should be (but rarely are) marked with red or black tape or paint to indicated that they are hot. The black wires from the switches are switched hot.
There is no neutral anywhere in the switch box. Typical arrangement is that power comes to (one of) the lights, and at that light you will find a white wire (that should be, but often is not, remarked red or black) connected to a black wire from the power feed leading to the switch location. The black wire in the same cable will then connect to the black wire from the light, and the white wire from the power feed will connect to the white wire from the light. The cable with the white connected to the feed's black will lead to your switch location.
Thus, power flows from the source, to the switch, back from the switch, through the load, and back to the source.