Speaking from experience: bifurcation is very uncommon and support is absolutely terrible, with most motherboards lacking the feature or having it be broken. Even when it is supported it's usually a pain...

Multiplexers just lets you switch between connected devices - it won't let you use both simultaneously. It's hot-swapping without having to physically change devices.

Bifurcation is much better supported these days due to the prevalence of mining.

And yes multiplexing is basically “time sharing” and the PLX chips operate on the physical layer only. PCIe switches have an internal bus, buffers and actual decode the packets to know where to send them too, they can also mediate between different versions of PCIe connected to the same switch so connecting a PCIe 2.0 device to a switch would not impact other 3.0/4.0 devices whilst a multiplexer would always operate at the lowest “speed”.

But PLX chips were pretty much the only thing you can get on consumer motherboards at least back when multi GPU setups were common and SLI/XFire motherboards we’re a thing (that said PLX did help quite a bit more with XFire setups than SLI due to the lack of an external cross bridge between GPUs in later XFire revisions).

The chipset on your motherboard can also support its own PCIe lanes however at least on Intel chipsets it’s not a classical PCIe switch its closer to the silicon in the CPU that runs the root complex.