To avoid confusion, this is technically not correct. The Thunderbolt protocol is effectively an encapsulated, serial PCI-e bus (leaving out yet further complications). Since nearly any adapter can be given a PCI-e peripheral connection, you can adapt everything from graphics adapters to USB controllers to DSP cards. USB itself, as a protocol, however, is NOT directly encapsulated by Thunderbolt 3 - it still requires a PCI-e protocol conversion, which means active hardware (e.g. a dongle), not simply electrical interface conversion (e.g. a cable).
I get that people are confused by the sharing of the connector, but that’s analogous to saying “you can carry data over wires”… it’s useless without specifying how in a myriad variety of ways (electrical signal levels, which wire means what, thence what the signalling pattern is to indicate data, ack, nack, flow control, error detection, thence the sequence of bits to identify, control, and communicate with each device on the bus or link endpoint, thence the control protocol for the device, and so forth). The connector is just a way for wires outside the computer to connect to wires inside the computer, absolutely nothing more.
Also, “Thunderbolt” as I’ve outlined above, is a specific set of meanings for those wires AND signals that go on them AND a method for devices to communicate to the computer AND some very basic layers of the methods for controlling them. USB is a completely different set of these core protocol specifications, plus MUCH MORE in terms of how the OS itself is aware of and generically provides driver support for certain classes of them.
While USB-3 and to a much greater extent USB-4 provide commonalities by which the appropriate protocol can be chosen and thus each is a little more aware of the other, they’re still by no means the same standard, they’re just closer to interoperability. USB-4 will be, if anything, even MORE confusing as it, as a protocol, is even more similar to Thunderbolt 4 (and they both have a 4 in their name, and share a connector, etc) but there ARE differences even there (Thunderbolt won’t provide backwards compatibility to any USB protocol, for instance, whereas USB-4 is a negotiated layer that can provide access downwards to them as fallback), and boy I can’t wait for the confusion here to start when cheap devices and dongles that really can’t translate between them well are abused for that purpose unknowingly.
In short, it really pays to know the differences between the protocols versus the connectors and what types of devices (and thus dongles) you’ll need to adapt what to what. But in near-universal cases, Thunderbolt “upstream” (e.g. on the computer end) can always adapt to USB “downstream” (e.g. towards the peripheral) but NOT the other way around.
Another takeaway of this is that you will always be able to adapt Firewire (downstream) to Thunderbolt (upstream) but it’s highly unlikely you’ll be able to generically adapt Firewire to USB-* (at least until USB-* becomes identical to a Thunderbolt-like generic bus) because Firewire itself is a very generic bus and does not translate to the command-orientation of USB protocols.
As people re-discover the joys of older audio hardware (the human ear hasn’t, after all, changed much), adapting older hardware will only become more confusing to the newer interfaces you’re going to see coming down the pipe, so best to at least comprehend this non-symmetrical relationship between TB-X and USB-X as it will drive the next generation of expansion ports.
