Tim Standing, OWC blog [Via TidBITS]:

Each pair of Thunderbolt ports on my Intel Mac is connected to a single bus powered by a dedicated controller chip. Since the two ports in a pair are connected to one bus, they share the Thunderbolt bandwidth. The total throughput of both ports together on a single bus is limited to 2,800 MB/sec.

And:

You can confirm the number of Thunderbolt buses in your Mac by viewing the System Report window (select About This Mac in the Apple menu and then click the System Report… button). Then click on Thunderbolt in the column on the left. Here is what I see on my 2019 16 inch MacBook Pro, showing the two Thunderbolt buses. (4 ports = 2 pairs of ports = 2 Thunderbolt buses).

Yup, confirmed, worth a look.

But on the M1 Mac mini:

System Report indicates that there are two Thunderbolt buses, one for each of the Thunderbolt ports!! This means that each port has its own dedicated bus and doesn’t have to share its bandwidth with any other port. Each one will have 2,800 MB/sec all to itself.

And:

The M1 Macs are the first Macs with support for the Thunderbolt hub feature found in Thunderbolt 4. Thunderbolt 4 requires computer makers to implement a whole set of features, most of which were optional in Thunderbolt 3. These features have actually been found in almost all Macs that Apple has shipped in the past four years, so the move to Thunderbolt 4 isn’t a huge change for Mac users.

These features include 40 Gb/sec Thunderbolt ports, support for charging laptops over Thunderbolt, and protection from malicious hardware that might try and snoop computer memory over Thunderbolt. The one feature in Thunderbolt 4 new to the Mac is the support of Thunderbolt Hubs.

The article goes on with some testing and details, along with a pitch for their own OWC Thunderbolt Hub. Hub needs aside, I found the bandwidth info worth knowing.