How so? In what way?
As
@Rob_TO has explicitly and
correctly pointed out, eTA is NOT a physical thing anyone can show to an airline. Visa-exempt travelers do not carry any document which states the traveler has eTA and, in particular, eTA is not something physically stamped in or attached to a passport.
What eTA means is that when the passport number and traveler's identity information is submitted to the CBSA system, electronically, a "board" response (as opposed to a "no board" response) is generated. The airlines may infer the visa-exempt traveler has eTA based on a "board" response.
Airlines may typically ask a visa-exempt traveler if they have eTA when there is a no board response, but that is their way of asking the passenger if they have obtained the necessary eTA (and to apply for it if not) or there is some other reason for the no board response. If the traveler has obtained eTA, again it is NOT as if the traveler has any physical thing to present which shows the traveler has obtained eTA.
Note: airlines have discretion to allow a passenger to board a flight despite a no-board response from the CBSA system. There are certain protocols or procedures for the airlines to follow in making this decision. But the decision to allow boarding or not is ultimately for the airline to make. Of course, if the airline allows boarding and it turns out the traveler is not admissible to Canada, there are compensatory and punitive consequences for the airlines. So it is usually anticipated that airlines are reluctant to allow a traveler to board in the event there is a no-board response from the CBSA system.
The "no board" response is NOT at all related to or akin to things like a "no fly" for security reasons list. Those are far more strict.