This is how mine looks on Big Sur 11.6.1 consistently, I didn't realize it wasn't normal for the Thunderbolt section to be there?! I have my Firewire interface plugged in via Firewire to Thunderbolt adapter and as long as the interface is on, the bus always appears.
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I don't have any hotpluggable devices to test the hotplug unfortunately.
I have noticed that the bus does not appear in the Thunderbolt section consistently when my interface is not plugged in. Sometimes the Thunderbolt PCI devices don't appear, or the controller's driver doesn't show as installed.
(I am happy that Thunderbolt is working consistently for my interface, as it frees me from having to add a Firewire card and take up a PCIe slot!)