For me it always sounded like USB4 was just another name for TB4 and they took over the complete technology.
That's basically true. See my earlier post:
Thunderbolt 4 basically is an implementation of USB4 - but with higher minimum requirements and a lot of the optional parts of USB4 made compulsory + an Intel certification program.
...and the new 80Gbps version is actually
USB4 2.0 - Thunderbolt 5 is an implementation of that - again with stricter minimum requirements and an Intel certification scheme.
Maybe they should invent a new connector type just for TB5, to get rid of that chaos, at least for new stuff?
Yup - then they could drop all of the USB-C connectors as "legacy-
so last year" and force everybody to buy Thunderbolt 5 to USB-C dongles and docks.... yay!
But seriously - the role of Thunderbolt, since TB4, is really as a branding for "USB-C with well-defined capabilities" to take some of the guesswork out of the USB-C committee-bound dumpster fire (where USB-IF couldn't even ensure that ports and cables used the appropriate icons to show their capability)
Is it theoretically possible to add any PCIe card to TB with an external slot or something like that if there are PCIe drivers? Should that work with the cards you can put into a Mac Pro now?
Not even theoretical - it is real:
https://www.sonnettech.com/product/legacyproducts/echoexpresschassis.html - there's also a list of supported PCIe cards somewhere on that list. However, the Mac Pro offers a
lot more bandwidth, including full-width 16 lane PCIe slots and the newer PCIe 4 standard.
TB5/USB4 2.0 supports PCIe 4 - so it should be better for the job. However, eGPU support requires Apple to do a U-turn on supporting non-Apple Silicon GPUs - they dropped/blocked NVIDIA support years ago and support for AMD GPUs ended with Intel machines.
Is Thunderbolt just external PCIe? Or does it only work similar?
The Thunderbolt/USB4 protocol is a sort of "tunnel" that can carry a mixture PCIe, DisplayPort and USB 3.2 data from the host to the peripheral. The TB/USB4 controller in the peripheral turns it back into physical PCIe/DisplayPort/USB connections.
Similar spec ones can be one tenth the price.
You need to check the "similar specs" bit carefully - the Apple ones support TB4 data rates, 100W power and the 3m ones are active cables with 'cable driver' chips to enable TB4 speeds over that much cable. They
are expensive but last time I looked it was more like 2-3x, and the 10x came from false comparisons with charge-only cables or 15W max USB3-capable ones.
Keep in mind the thunderbolt controller is now part of the SoC. Apple cannot introduce a new Thunderbolt with their M4 Macs even if they wanted to now because these ports are wired straight to the SoC and the M4 chip has a TB4 controller. The M4 iPad Pros had manufacturing dates going all the way back to December 2023,
Thunderbolt 5 is an implementation of USB4 2.0 (with some optional bits made compulsory) which was published in 2022 (...and Apple are part of the USB IF, so it wouldn't have come as a surprise...)
So -
although I'm not claiming that M4 does have USB4 2.0 - you can't rule it out just from the timeline, and it could still turn out to be a distinguishing feature of the pro/max variants when they appear.
Also remember that we've seen cases where M1/M2/M3 MacBooks only claim to have have TB3 while Mac Minis with the exact same SoC have TB4 - simply because the MacBooks have an internal display and can't support the two TB-connected external displays required by TB4 (...and then the software patch to support clamshell-mode on M3 upgraded the port to TB4 in all but name - I guess it's just Intel certification standing in the way). I don't think the iPad Pro even qualifies as TB4.