Most computers from 1990 didn’t have built-in ethernet or native support for TCP/IP, though, so you would have to buy and install both of them.
edit: I phrased this poorly, all Macs didn't have it as standard but what I meant to say is they started releasing Macs that had it as standard, starting from the mid-range (Centris) and up, the point is just Ethernet had stopped being a niche technology
The point is that the first 10 base T still works if used with modern equipment. That standard is from 1990 and grew extremely fast.
Around 1993-1995 coax was considered totally outdated. Fixing termination problems where never fun. A nail in the coffin was 100 Base T which was standardized in 1995.
Tokenring and ATM was contenders around that time but lost to ethernet.
That kind of backwards compatibility is really amazing.
From what I can tell, iMacs have consistently come with Ethernet after around 1998 and Powebooks starting in 1999.
The PowerMac 6100 very much did have Ethernet built-in, in the form of an AAUI port (you needed a transceiver to convert to either BNC or 10Base-T since at the time it wasn't obvious which people wanted)
I remember even back then you could get an Ethernet card for a PC cheaper than that.
The principle of the AUI port was that the three Ethernet connections (10Base5, 10Base2, 10BaseT) could all be used with a 'cheap' change of "MAU" adapter. For 10Base5, it would have been impractical to have the thick cable brought right up to the computer.
The 10Base5 MAU was relatively expensive, I see a card sold with it costing £50 more than the same card with a 10Base2 interface in a price list from 1993 [1].
I don't see a price for a 10BaseT interface alone, but it should be much cheaper — it's just a couple of low-pass filters and some isolation. Even back then I think these main components were sold as a single package ("chip"), so the 10BaseT MAU is two plugs, one "chip" and a couple of capacitors.
Anyway, Acorn¹ computers were sold from around 1991 with an optional Ethernet card. Basic TCP/IP was included in ROM on the card, and it works fine between my Linux and Acorn computers.
[1] https://chrisacorns.computinghistory.org.uk/docs/IntInt/II_P...
¹ Acorn as in "Acorn RISC Machine" → "Advanced RISC Machine" → the ARM chip in your phone.
Though that counts as a "PC" only if you were rather rich.