The cable was chewed through by cats, so perhaps it was three just in that moment.
The connection was overall unreliable, so I guess it must have been four, just not all of the time.
Ah, the old Cat-3 cable. Been there.
100BASE-TX uses just two pairs (lanes), one for sending and one for receiving. 1000BASE-T uses all four pairs, for both sending and receiving. Therefore, a 100BASE-TX interface that's only receiving needs to power up one pair. A 1000BASE-T interface needs to power all four pairs all the time.
I recall reading about some extensions that allow switching off some of the pairs some of the time ("Green Ethernet"), but I think that they require support on both sides of the link, and I'm not sure if they are widely deployed.
For regular Ethernet, the switch will have a table of which IPs are on which NIC and thus can dynamically send packets at the right transmission protocols supported by those NICs without degrading the service of other NICs.
100m is fine. 10m is fine but I can’t think of anything that negotiates 10m other than maybe WOL (I don’t use it enough to be sure from memory).
If I didn ahve something esoteric it would be on a specialised vlan anyway.
It's not, cf. sibling posts. The GP probably learned networking in the 80ies~90ies when it was true, but those times are long gone.
(unless you're talking wifi.)