How would I go about putting my device in maintenance mode? Iirc that was only available for repairs at Samsung Authorized stores?

I truly don’t understand why any tech would ever do any snooping.

You don't understand that some people are just dirt bags?

Shitty people will do shitty things. That said, if you don't give your password, be prepared to have the technician test all sorts of stuff in front of you. The selfie camera, ear speaker, microphone, etc. sometimes are mounted on the screen. If there are problems, the tech will need to redo the repair. Not advocating for giving your pw, but be prepared for the process to be less convenient.

Edit: My bad, should have clarified I'm talking about phones exclusively. If you're worried about your computer, create a non-admin user and give them that password. If they had the skills to bypass that, they wouldn't be working at a repair shop.

If someone has physical access to your device, they also have the ability to access your files without your password. Unless you are using sophisticated full disk encryption, but that makes it more time consuming to gain access.

You almost always need to the password to test a phone thoroughly. You can see that the screen works on the lock screen, but what about the front facing camera, and secondary microphone that are attached to the screen and need to be transferred, or replaced if you do it like Apple. On newer iPhones the slightest defect can cause face id to not work. On laptops it depends. Sometimes live USBs don't have the right drivers to test all the hardware. When you assume things are simple you're usually wrong.

People are allowed to leave shit on their private computer out in the open and IT bros have 100% of the moral responsibility to not look at it.

It's unrealistic and more importantly unjust to blame the victim here.

if you are technical enough to replace a hard drive then when you buy a computer also buy an extra drive. day1 build your machine or recover to the new drive. keep original drive in case of repair need. it also helps to troubleshoot if your problem is hardware or software.