I bought my first iMac from an independent Macintosh store. There was no Apple store here at the time. After 3 years (about a month after the extended warranty would have expired if I had bought it, which I didn't) the Ethernet died and I could not connect to the internet. I went to the Macintosh store and they told me that in the Mac, the Ethernet is built into the logic board, which they told me was Apple-speak for the motherboard. They quoted me a price for a new logic board which was about $200 cheaper than a whole new computer. They told me the only fix would be a new logic board or a new computer.
I went home and did some on-line searching, and I found a USB-to-Ethernet dongle for $40 or $50, don't remember exactly. I bought it, plugged it in, and never had any trouble. Repeated speed tests showed the same throughput as I had been getting before the Ethernet in the computer had died. Eventually I retired that iMac and bought a new one because of software issues as it couldn't run anything past OS X 10.6.8.
I do not know if the people at the Mac store were ignorant of the fact that you could buy a USB-to-Ethernet dongle for under $50, or if they were crooks trying to scam me for a couple of thousand dollars. That store went out of business and for a couple of years there was no store locally where you could buy Apple products, until the Apple store opened downtown.