The USB 2/USB 3/USB 3.1/USB-C/Thunderbolt/Thunderbolt 2/Thunderbolt 3 deal

Thunderbolt 1 and Thunderbolt 2 use the same type of ports and cables.

Thunderbolt 3 uses the USB-C type of port and cable.

USB 1, 2, 3 and 3.1 use the same generic type of USB port and cables.

Thunderbolt 1 and 2:

USB 1, 2, 3, 3.1

Thunderbolt 3 / USB-C

You cannot plug Thunderbolt devices into USB ports, but you can plug USB devices into Thunderbolt ports via an adapter.

Thunderbolt carries more data in its signal, than USB so a USB connection cannot be up-converted into a Thunderbolt connection.

Speed is determined by the slowest component in the connection. A USB 3.1 port can be used to connect a USB 1 or 2 device but speed will be limited to USB 1 or 2 speeds. A USB 1 port can also take a USB 3.1 drive but again speed will be limited to the USB 1 speeds. USB is pretty universal.

As to the External Drive enclosures, generally external drives are just boxes with a regular internal computer hard drive. These boxes are known as enclosures. The internal hard drives in them can generally be moved into another box. Though some comercial external drives are very very difficult to take apart.

You can buy newer faster enclosures for the internal drives to connect them to faster types of ports. Though its generally not something I would be recommending unless you are processing a lot extremely time sensitive data from the drives. As disassembly of the enclosures as said can be very difficult. Sometimes requiring the destruction of the enclosure.

USB Type-C™ is a specific feature of a product, it is not a generic description. Think of “USB Type-C” as an adjective not as a noun.

About Thunderbolt 3

MacBook Pro (15-inch, 2016), MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports), and MacBook Pro (13-inch, 2016, Two Thunderbolt 3 Ports) come equipped with Thunderbolt 3, an I/O technology that connects devices to your computer at speeds up to 40 Gbps. Thunderbolt 3 combines data transfer, video output, and charging capabilities in a single, compact connector.

The Thunderbolt 3 (USB-C) ports on your MacBook Pro support Thunderbolt 3 devices at speeds up to 40 Gbps when you use a Thunderbolt 3 (USB-C to USB-C) cable. You can also connect Thunderbolt and Thunderbolt 2 devices using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter at maximum speeds of 20 Gbps. Learn more about Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter compatibility.

from:

Connect with Thunderbolt 3 on your new MacBook Pro - Apple Support

ThunderBolt-3 is an uncommitted computer port that can do almost anything, with the right cables/adapters. Your choice of cable/adapter specializes the port to one particular purpose, usually more narrow than its native capability.

Questions

1) On the MacBook Pro 15.4" mid 2017, what do they mean by "Thunderbolt 3 via USB Type-C"?

Thunderbolt 3 uses the same connector as USB-C.

2) Which ports can I connect my USB 2.0 external hard drives (whose ports are, I think, Micro USB B and USB Mini-b (5-pin)) to: Thunderbolt 2, Thunderbolt 3, USB 3.0, USB 3.1, and/or USB Type-C ports? Is there an adaptor for any of the remaining connections?

If I were you I would get rid of USB 2.0 enclosures and replace them with those supporting USB 3.x.

3) Which ports can I connect a USB 3.0 male end into: Thunderbolt 2, Thunderbolt 3, USB 3.0, USB 3.1, and/or USB Type-C ports? Is there an adaptor for any of the remaining connections?

Only USB 3.0 female ports. USB-C use USB-C ports. A USB 3.0 connector can be used with an adapter to connect directly to USB-C ports. You can also find multi-port USB 3.0 adapters that will connect directly to a USB-C port.

4) What male-end of the Thunderbolts and the USBs is most compatible with the ports USB 2.0,Thunderbolt 2, Thunderbolt 3, USB 3.0, USB 3.1, and USB Type-C?

Thunderbolt to Thunderbolt cables. TB2 to TB2 or TB3 to TB3. Neither is compatible with USB 2.0. TB3 uses the same connector as USB-C.

5) Which is the fastest? Thunderbolt 3, USB 3.0, USB 3.1, or USB Type-C?

TB2 is as fast as USB 3.0. TB3 is faster than TB2 or USB 3.0. USB-C is as fast as TB3.