Can't figure out the origin of loud screeching noise

I’ve had my bike for over 1 year. Since the day I bought it it’s had this problem where it makes a really loud screeching noise coming somewhere from the back. I didn’t notice it at the store because it comes and goes. It started shortly after I bought it and I brought it back to the store and asked about it. They said it was caused by the paint on the rims and it would disappear with wear. It’s been one year, the rims are somewhat worn, and it’s as loud as ever. In fact the front rim makes less noise and is much less worn. The front rim also makes some noise but a lot less than the back. If anything the back noise seems to be getting worse.

I know someone who bought the same model and she’s never had this problem.

It doesn’t make much difference whether the bike is dirty or clean, although it happens more frequently if the bike is dirty, along with other creaking noises.

I’ve tried loosening the bolts on the back wheel, makes no difference.

It’s not because of lack of oil, or excess. I found some oil had gotten into the back rim and I scrubbed it thoroughly with alcohol. The noise got even worse.

It’s not the brakes because it doesn’t make any difference whether they are new or worn out. Loosening or tightening the brakes makes no difference. I’ve tried with 2 different models (the first the same as the one on the front), no difference.

Tuned or untuned, makes no difference. I’ve cleaned it thoroughly, no difference.

Pedaling or not doesn’t make any difference. This sound only happens when I brake, otherwise the bike works normally. The brakes work well, except for this issue.

Three things that make a difference:

The sound gets worse if I ride through puddles and the rims get wet. Gets better when they dry.

My weight. I weigh ~55kgs, it’s a medium size bike so there shouldn’t be a problem. Although, if I run alongside my bike and hit the brakes, no noise. At all. I tried the same thing but I pressed down on the seat with my hand when I braked. There was some noise, but not much.

Another thing that made some difference was flour. Yes, I put some flour on the back rim just try it. Reduces the noise a bit, but not much. I read that scrubbing it a bit with sand paper could help, I want to try that. Would dirt help? Possibly, but even though I go on a lot of dusty trail rides not much dirt gets on the rim.

Photos: https://imgur.com/a/AF05KHy

Can anyone help me figure out the problem? If all else fails I will just buy new rims.

EDIT: I think I have figured out what it is. I’ve followed this video:

and I’ve been able to eliminate the noise almost completely. I will try it again tomorrow when I have more time.