I have an unused junction box on the ceiling that I would like to install a light fixture (1 white and 1 black wire). When I open the lid, the junction box has 1 white wire with no ends (just running from one end of the junction to the other end of the junction), and a yellow wire that also runs from one end to another, but there's one open end. Do I need to cut the white wire in the middle and connect my light fixture's white wire together? And the black to the open end yellow?
Yellow is a hot wire color, same as black.
In conduit wiring methods, white is always neutral.
Given that they left you a single yellow wire that ends here, that is clearly the switched hot to the lamp.
Given that one of the neutrals passing through this box was given a generous loop, that is clearly the correct neutral to be used with the solo hot. (It matters). Cut it at the halfway point and pigtail to your lamp. It needs to go through, because it’s also returning current for other stuff.