You're familiar with how they all begin: "this video is sponsored by Squarespace." Ok, well, after that, they then feed you the next line, almost all of it verbatim, "I am not sponsored by (insert brand here). They have not paid me to do this review or told me what to say about it. This review is entirely my own and they don't know I'm making this video. I was/was not provided a sample for the purposes of this review. I am not biased in any way so you can trust what I'm about to say about this product."
What they don't mention, obviously, is if they had to return that "review unit." But if you pay close attention, you may catch them using the product in later videos, especially if they were especially laudatory in their review. At least they weren't paid, right? Wrong.
Payment is simply one form of compensation. That "review unit" usually comes along with an email that says, "you don't need to send this back to us." Simply put, the item itself is payment. Just because you didn't get to choose the item, the form of compensation or were given a special title associating you with their brand doesn't mean you weren't paid. They paid you with product and exclaiming otherwise is a flat out lie by Youtubers who persistently try to claim otherwise. Rarely are these items cameras and lenses; they're usually "soft goods" or accessories, like bags, lens filter kits, etc. that would likely get damaged during a thorough review period or whose cost of manufacture and/or retail price is so low that the cost of return, reconditioning and resale would consume all or more of the potential profit.