Sometimes it goes deeper than just looking for "Thunderbolt capable"
Thunderbolt just means that that external device has the ability to send and receive data over that particular cable.
The speeds at which it is capable of and actually doing can be on opposite ends of the spectrum.
Examples:
If you have a Thunderbolt 1, 2, or 3 external hard drive. You will only get speed based off of the slowest link in the chain. If it is a platter based hard drive you are looking at 120 MegaBytes per second maximum, even if it is in a Thunderbolt 3 enclosure.
If it is a SATA 3 based Solid state drive. Now you are at 550 MegaBytes per second maximum. (give or take a few MBps)
So even if Thunderbolt 3 is capable of 5 GigaBytes per second, you will not ever see that speed unless you have actual hardware capable of even transmitting and receiving at that speed.
Also. Say you have a computer with an internal hard drive. Platter. So it will read and write at approximately 120MegaBytes per second. If that computer has Thunderbolt 1 or 2, let's say a Thunderbolt 2 external SSD, so maximum read and write is 550'ish MegaBytes per second. If you are sending or receiving data to or from the internal platter drive to the SSD, you are limited to the speed of the slowest link, which is now the internal hard drive so even though you have an 500+ MegaBytes per second capable device, if it has to send or receive from that platter drive everything slows down to 120 MegaBytes per second.
Another example.
I had purchased an external RAID capable Thunderbolt 1 enclosure several years ago. Thunderbolt 1 is capable of 1.25 Gigabytes per second read and write.
That drive only sent and received at 120 MegaBytes per second with RAID 0 Solid State drives.
Because that enclosure only had a SATA 1 controller built in, I was then limited to SATA 1 speeds on SSD's RAID'd in a Thunderbolt enclosure.
The whole thing can be quite confusing up front to consumers until you put in the time to learn where the limitations are in the system.
And then USB-C is it's own other thing.
USB-C is just a shape of the plug connector essentially.
Like Thunderbolt it is capable of very high speeds but can be limited down based off the the hardware connected.
Example with this that I've had experience with, not all USB-C cables are created equal. I have a handful that only transmit at USB 2.0 speeds, some that do USB 3.0 speeds and a couple that do USB 3.1 speeds.