iMac17,1 and USB C (Thunderbolt 3?)

Hi -

This is a stupid question, but...

I'm trying to connect the following:

27" iMac17,1 (5K, Late 2015)

3.3 i5

4 USB 3 Ports

2 Thunderbolt 2 Ports

With:

USB C storage device.

I have been to speak with my local apple store several times and have received completely different answers each time regarding the best way to connect these two items. I purchased (and am currently using a cord with USB C on one end and USB 3 on the other end.

The salesperson told me that I was crippling the connection by using the cord and he tried to sell me a Thunderbolt 3 cable ($49) and a Thunderbolt 2 (female) to Thunderbolt 3 (male) connector ($49). When I suggested that I actually needed the opposite connector (Thunderbolt 3 Female, Thunderbolt 2 Male) he insisted that his solution would work.

I came back and another "Genius" told me that there was no way to connect the two.

I do not believe the advice that I am getting from my local reseller. Is it as simple as connecting a USB C cord to the storage device and then using a simple USB C to USB 2 converter?

What would be my best way to connect the two? I'd like to have the maximum throughput if possible.

Why are there so many USB 3 connections? I see that there is a USB 3.1 5g/s, and a USB 3.1 10g/s but I can't figure out what is on my mac. I also see that Thunderbolt 3 is 40 g/s, but that it uses the same USB C connection. Apparently USB C could be 40 g/s, 10 g/s or 5 g/s.

My "friend" at the apple reseller said that USB C and Thunderbolt 3 are exactly the same. Does that even make sense?

So, bottom line, I'm trying to figure out the best way to connect USB-C storage to my iMac17,1

Any suggestions?