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I was in the process of replacing a 23-year old fluorescent tubelight fixture in my garage with a similar (physical) size LED fixture. When I opened up the light, I found that from the ceiling box there were three wires (none of which was a ground wire) coming out. Picture shown below.

Two of these (left and middle) were combined with a wire from the fixture using a twist connector. And another one from the box (right) was connected to another wire from the fixture.

enter image description here

When I turned the mains back, I find that the wires on the two extremes were both hot wires and the one in the middle is a neutral. And, both these hot wires are both hot even when the switch (the one that operated the fluorescent lamp) is turned off.

About a foot away on the ceiling is the plug to my garage door opener. When I removed the light fixture, the garage door opener stopped working and in fact that plug does not have power.

I did a quick test -- connected the hot (right) and neutral (middle) to the new LED fixture and it works perfectly. The hot on the left was just capped. The garage door plug did not get power.

QUESTION -- should I connect it back the way it was (description above) and are these connections making sense? Why is a neutral and a live being connected here?

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Turn the power off. Connect the middle wire to the left wire with a wire from the fixture (white if there). Then connect the right wire to the fixture. Turn on the power and check for correct operation. If there is a ground wire with your new fixture, connect it to the bare copper wires in the back of the junction box.

You're not connecting a neutral to a live wire... you're closing the loop from your garage door opener

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It's not a live wire at all. It's a neutral. This part of the circuit doesn't just serve the light. It also carries power (hot and neutral) onward to other loads.

The always-hot is carried from wherever (switch, perhaps?) on the black wires tucked in the back there.

The switched-hot is carried on the red wire, obviously you want the lamp to take its hot from the switch, so it isn't on 24x7.

The neutral wires brought

  • Neutral from supply
  • Neutral onward to other points of use
  • Neutral to the lamp

You only connected neutral from supply to the lamp, so neutral was severed to the onward points.

Electrical current flows in loops. Power is going out the hot wire to those onward points of use, but then is unable to return via the neutral you severed.

You see "hot" because electrons are queued up there waiting to return via neutral. This is normal when a neutral is disconnected; it's one reason neutrals need to be treated as hot wires from a safety POV.

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    "[...] it's one reason neutrals need to be treated as hot wires from a safety POV" — learned that the hard way. Won't forget my lesson. – SQB Sep 2 at 9:48

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