If you are speaking about using wifi at your house, the principle is the same. There is a wireless router between you and your ISP. Whether you plug a network cable in or use wifi, the device that the ISP sees is the modem that they gave to you. They know that that MAC address corresponds to your account. No matter how many devices are behind that modem on your home network, every bit and byte of data sent to or from your house can be tracked to that modem.
If you are at a public wifi, that's a bit trickier for them to track in some cases. BUT, your wifi adapter on your laptop, cell phone, iPad, or whatever has a MAC address, too. The manufacturer can track who purchased each device by that MAC address.
As mentioned previously, you can fake your MAC address on your computer but this is probably not something everyone will want to attempt.
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