Trying to add a light fixture to a box that used to have a ceiling fan

[crownpr1nce]

Here is a picture of the ceiling box

In the kitchen we have recessed spotlights and there used to be a ceiling fan, and there is only 1 switch with a dimmer. We removed the fan and wanted to add a Bazz disk to sort of fit in with the spots. I opened it up, connected it in every possible combination and nothing works. Here is a summary of what I figured out:

- The black from OLD and NEW have to be connected together or the spots won't work at all

- I connected the 3 whites and 3 grounds together and never touched them. Assuming that's correct

- I tried every combination with the 3 black wires (OLD group, NEW group and light ficture) and red wire, nothing worked.

- If I connect the light fixture to NEW black, it is always on regardless of what else is connected there

- If I don't connect it to to NEW black, no matter the combo, it won't turn on at all

I tested the wires with a contactless wire tester:

- NEW black and NEW red seem to be always live even when the switch is off.

- OLD black seems to be live when the switch is on, but its unclear. The tester beeps a few times and stops. If I move it a bit it'll do the same. Worth noting I think the wire is aluminium (old house)

- When I pass the tester over one of the spot, it beeps whether the spot (and switch) is on or not.

- White and ground have no readings

I am completely stumped. No idea what to do or what is going on... I re-wired it OLD with NEW black and took off the fixture for now. The spots work right now.

Ask any question if it can help!