Rear brakes noisy

Did you replace ALL the pads on BOTH sides? They are usually sold as sets of 4 so use all 4. Did the pistons in the callipers push back smoothly? Sometimes a 'sticky' piston can cause a pad to rub causing noise.
Is the sound constant or does it pulse under light braking? Pulsing can possibly be a warped rotor 'tapping' the pads as the the warp passes them. The warp can be tiny and you'll need a specialist tool, (A Dial Gauge Indicator), to measure for it.
Deeper tone noises can suggest it is the rotor causing the issue.
Commonly rotors can last between 30,000 to 70,000 miles depending on how hard they are used. Did you check the condition of the rotors when you changed the pads. On a 2015 model there is a chance these are the factory installed discs. So it could be worth you changing them anyway.

Some tires are noisy.
Some rear axle HD differentials make noise.
Differential bearings may be noisy.
When the differential was drained, the oil should have been inspected for metal residuals.

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The job wasn't done correctly,that is obvious

Most places don't remove any rust so parts slide
as they should ,nor do they lube anything

I have worked on brakes for 50 years come july & never
had a noise or hot,no issues

When you say changed out,that's what you got
It is called hanging pads,slap them on & done.

You have brake hoses & claipers to consider,parking
function,etc

If they don't release for some reason,you have heat & noise
Sloppy work

Make sure the mechanic checked the park brake shoes inside the rear rotors, they quite often break away from the lining and get wedged in the rotor, sometimes causing a constant squeal noise. I've also seen the brake pad anti-rattle metal clips become loose and wander in the caliper bracket, they lightly rub against the rotor and cause a squealing noise similar to worn brake pads. They are also very bad for the backing plates rusting out prematurely and rubbing on the inside of the rotor. Other than that he can just try to champher the edges of both front and rear pads to help dissipate the brake dust, from collecting and causing squeak noises.

Make sure backing plates behind front & rear rotors aren't rubbing on rotors. Jack up each wheel individualy & spin. May also be emergency brake shoes rubbing inside drum inside rear rotors.

probably a dumb question, but if there is a backing plate, did you make sure it isnt bent and hitting the rotor? you never want to sand new rotors, although i dont think it would cause this concern. i would suggest lightly scuffing up the brake pads. this will eliminate any noise caused by the contact of the rotors and pads. if it goes away then you know its a problem with the pads/rotors.

scoring of only the inside rotors would make me think that possibly the caliper slides are frozen up. make sure they move quite freely on the slides. it is not uncommon for brake pads to score rotors though.

but brake noises at low speeds that dont' change when applying light brake pressure are usually some sort of metal or something contacting the rotor.

If it is really dragging, then there will be a lot more heat then at the other wheels. Careful, though, the rotor can be hot enough to burn you if there really is drag. The rears also have the parking brake cable as well as hydraulics. So have someone pull the parking brake lever on and off, while you watch underneath to see if the cable is releasing correctly. There could be a sticking calipher that may need to be freed up. Warranty should cover it.

hi make sure the hub flanges are clean and no rust

Well make your power steering pump is fucked by the sounds of it.