From what I can discern, the link you gave shows an enclosure which only operates a 10 Gb/s through USB-C. Your 2015 iMac does not have any USB-C jacks. You may need this enclosure instead, which can be plugged into a USB-A jack at 5 Gb/s. (I suppose perhaps maybe the only difference between the two is the more expensive enclosure comes with a USB-C to USB-A cable.)
To get 10 Gb/s, you would have to invest in a powered dock. An example would be this OWC Thunderbolt 2 Dock Solution.
What you would need to get to 10 Gb/s is shown below.
Frankie's Computer
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Thunderbolt-2 to Thunderbolt-2 Cable
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Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter
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Self-powered Thunderbolt-3 Dock
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USB-C to USB-C Cable
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Frankie's Hard Drive Enclosure
Apple did not offer any iMac models in 2016. The 2017 and newer iMacs do have a USB-C port which can support a 10 Gb/s USB drive.
Actually, your 2015 iMac could get 20 Gb/s maximum speed through the cables by using a Thunderbolt 3 enclosure, as shown below.
Frankie's Computer
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Thunderbolt-2 to Thunderbolt-2 Cable
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Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter
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Self-powered Thunderbolt 3 Drive Enclosure
Note the following from the Apple website About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
This adapter is bidirectional, which means you can use it to connect Thunderbolt 3 devices to a Mac that has a Thunderbolt or Thunderbolt 2 port. In that case, the Mac must be using macOS Sierra or later, and the device using Thunderbolt 3 must provide its own power.
So, since the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter from Apple cannot supply power to the Thunderbolt 3 enclosure, the enclosure would need to be self-powered. For example, there is this Trebleet Thunderbolt 3 Four-Slot M.2 NVMe SSD Enclosure. The website states the enclosure works with Thunderbolt 3 (USB-C) to Thunderbolt 2 adapters such as the one made by Apple. A cheaper example would be this Sabrent Thunderbolt 3 To Dual NVMe M.2 SSD Tool-Free EnclosureSabrent Thunderbolt 3 To Dual NVMe M.2 SSD Tool-Free Enclosure. Note that the website does not mention if the enclosure works with a Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter.