Trying to understand this light wiring

This is an exterior light, no motion sensor.

Cable 1 (2 hots, 1 neutral, no ground) runs from the main panel (2 handle-tied 15A breakers), in metallic conduit up to the bottom of this junction box.
<img src="https://i.imgur.com/WNiAxZl.jpg" width="3024" height="4032"/>

Cables 2 and 3 connect to Cable 1 inside the junction box. Cables 2 and 3 run into a junction box in the attic (I think, from last time I was up there). Cable 2 has 2 hots, neutral and ground. Cable 3 has hot, neutral, ground. Both Cable 2 and 3 get their power from Cable 1 (with the handled tied breakers shut off, everything in this junction box is de-energized).
<img src="https://i.imgur.com/oH7UM44.jpg" width="3024" height="4032"/>

The red wire from Cable 1 and red wire from Cable 2 are spliced together. No red wire to the lights.
<img src="https://i.imgur.com/kEp62vO.jpg" width="3024" height="4032"/>

The neutral/white from Cables 1 and 2 are spliced along with whites/neutrals to lights.
<img src="https://i.imgur.com/ADnuj7M.jpg" width="3024" height="4032"/>

The black wires from Cable 1 and 2 and the neutral/white from Cable 3 are spliced together.
<img src="https://i.imgur.com/WNiAxZl.jpg" width="3024" height="4032"/>

The black wire from Cable 3 is spliced with the blacks to the lights.

Again, no motion sensor on this light assembly. This is on the same circuit as another external light that does have a motion sensor. However, as far as I can tell, when the switch for these lights (they share a switch) is on, this light stays on (the light with motion sensor goes off after initial power up). This seems strange to me - why have an external light that is steady on, on the same circuit and switch as an external light that has a motion sensor? The house has other motion sensor lights as well, so it really seems odd that the plan was to have this one external light (on side of house, with no doors on this side) stay steady on. At first I thought it must be wired to rely on the motion sensor from the light assembly on the same circuit, but that light assembly has only 1 hot wire running to it. There is another light assembly further away with motion sensor that isn't working (tough to reach, so I haven't yet checked whether issue is dead bulbs, dead motion sensor or just loose wiring, and whether it's on same circuit - I suspect it is).

Why is it wired such that Cable 3's white is spliced with Cable 1/2 hot? So with breaker on, the white is always live, but the black is only live when the switch is on. Is it because the light assembly is before the switch in the circuit and they only ran 2 conductor cable from switch to light assembly?

I want to add a motion sensor here. So I guess I can keep wiring exactly like it is and run hot from Cable 3 and neutral from Cable 2 to the motion sensor?

Separately, there is no grounding screw or tapped hole in this box, so it looks like the ground wires from Cables 2 and 3 were just wrapped together and bent so that they made contact with inside perimeter of box.
<img src="https://i.imgur.com/sX9Yjzj.jpg" width="4032" height="3024"/>

To correct this grounding situation and provide a proper ground fault current path for Cables 2 and 3, is the easiest option replacing this box? What I referred to as Cable 1 appears to be individual wires (2 hots, 1 neutral) run inside flexible metallic tubing from main panel to junction box inside house, then into LB then metallic conduit outside the house to junction box with the lights. Maybe I can fish a ground wire through by pulling the existing wires out at the LB trailed by pulling wire, and then pull existing wires and new ground wire back into place?

 

The white is connected to the hot wires because there is a single cable going to the switch. This is called a switch loop. The white is hot and the black is the switch leg. In fact, I think I can see the white and black are from the same cable which confirms it is a switch loop.

You can test by taking a meter set to volts and see if you get 120 volts from hot to the metal. If so it is likely grounded. You can also follow the conduit to see if it is a continuous wiring method between the box back to the panel.

If the box is grounded then all you need to do it drill/tap a 10-32 hole and install a ground screw. Or you can replace the box which will already have a threaded hole in it.

 

Thanks Tolyn. Yeah, you were correct re: switch loop. I was thrown off by the 2nd hot/red wire, I figured that had something to do with this lights, but it does. I checked the switch wiring and it's just hot and neutral. 2nd hot must be for something else in attic, perhaps the attic fan which is temperature sensor activated.

I just replaced the old junction box since I don't have any taps, with a new box with ground screw. Light installed and working fine.