We see things like correctly terminated. And the pair will have an impedance, and the twisting of the pair is important to have that impedance, due to starting with two then four then eight the pairs are not as expected, there are different plans but this
is typical note the green - green/white pair. It is the same with telephone cables, the twist is important.
The telephone has just two wires, and the polarity swaps to ring the bell, when it reaches the house a network is used with capacitors and resistors to store the energy and to enable the bell to ring, your allowed 5 REN or 5 one REN bells, however cordless phones don't need the network to ring the bell, there was the option of two types of ring, using two numbers, one would auto switch on the fax machine, which would in turn turn off the telephone to get a better signal.
When broad band came in we started using band pass filters to separate broad band and telephone, but internal phone lines became a problem, as for a phone using brown and blue would work, but for digital it would not, and I know with my own house to get a reasonable speed all the original wiring for phones had to be disconnected. It worked fine in the days of fax and phone, but not broad band.
With last house I did install some LAN cables, but the LAN socket
tends to group the wires together so for A it shows 1, 2, 3, 6 for example not 1, 2, 3, 4 which one would expect, my simple tester will show if all wires connected and if 1 goes to 1 etc. But it is very easy to end up with a brown - blue pair instead of a brown - brown/white pair, and the tester will not highlight the error.
The LAN still works with the error, but speed is reduced. As yet this house is all wireless, I intend to fit a LAN, but still on my todo list, in the main due to problems with the broad band and the possibility the point where it enters the house will change.
My son in his house fitted a server in the loft and loads of LAN sockets, intention was to have no wireless, however so many items are not available in a wired version, my TRV heads can't be hard wired, my smart sockets can't be hard wired, and the list goes on, so even if I do like my son and hard wire loads of LAN sockets to a switch, (switch is one step better than hub, but not as good as a router) still need wifi.
So there is a limit and not sure the point in having a speed between PC's which is better than incoming broad band.