Need neutral wire on "smart z-wave" wall switch - don't have one

To preface this, I’m experienced with wiring up AC outlets/switches/etc – but when it comes to looking at work someone else has done – this is what’s throwing me for a loop. And maybe someone else more experienced will recognize it.

When we moved into our home – and this is all from my memory so may not be 100% accurate – but in this room I believe there was no light switch. We wanted to install one, and add a fan to center of the ceiling. Contractor did this for us, by putting in one of those drywall blue boxes for the switch. We want to upgrade this to a "smart" z-wave style switch – that has two terminals for the load on one side, and then a "traveler" (for 3-way, which we don’t need) – and then a neutral terminal.

Here’s what I found inside the box:

I see a black and white wire, and a ground coming from the left side of the box. The white and black are twisted together and capped off. Another piece of Romex comes in from the right, has it’s white capped off, ground tied to the other ground, and the black and red wires go to a light switch.

If I use my Klein voltage tester, the black wire going to the switch appears to always be "hot". The black and white twisted pair also shows "hot". The shorter white wire from right Romex shows nothing as far as I can tell.

I haven’t climbed up to rip open the wiring harness for the fan, but I guess I can if I need to.

I tried to connect a lead from the black/white twisted pair into "neutral" on the new switch – and it worked to turn on the light but then would immediately shut off.

Am I stuck here/unable to put a smart switch in this location without some additional work? I would hope my contractor had done something "typical" in this scenario, but maybe he didn’t.

Thanks for any guidance/direction. And yes, I asked the contractor but he does not recall how he did it (he’s getting kinda old).

Here’s an enhanced picture of back. Left side is black and white and ground wire. Right side is Romex with black, red, white and ground wires.