Can an NFC tag unlock or start your car?
No. A phone that opens a car holds a credential issued by the vehicle manufacturer and kept in protected storage that applications cannot write to. A blank tag is memory you can write freely, which is precisely why it can never become a key. The two are different objects.
Two different objects
A tag is memory. It answers when powered, returns what is written on it, and has no way of refusing anybody.
A credential is a secret that proves itself without being handed over, held somewhere that will not disclose it, and issued by a party that decides who gets one.
Both can be presented to a reader by holding something close. That shared gesture is the entire source of the confusion, and it says nothing about what happens next.
What a phone key is
A phone that opens a car was given that ability by the manufacturer, through their own enrolment process, and the credential lives in a protected part of the phone that ordinary applications cannot read or write.
Digital keys are specified by the Car Connectivity Consortium, whose members include vehicle and phone manufacturers. Enrolment runs through the manufacturer, and the credential sits in a secure element rather than in a record an application can write.
Which vehicles support this, and by which scheme, changes by manufacturer and by model year, and the answer belongs to the vehicle documentation rather than here.
What matters for this question is the part that does not vary: the key was issued, not written. There is no export of it, and no application that can produce one.
What a tag can do near a car
Everything except open it. Start a navigation destination, set the phone up for the drive, log the mileage at a handover, carry the details of who to call after a breakdown.
A tag can also hold information about the vehicle for whoever is standing next to it, which is a genuinely useful thing to put in a glovebox or on a windscreen.
None of that touches the locks, and none of it needs to.
Why the answer will not change
A car that could be opened by something anyone can write would be a car anyone can open.
The whole value of the credential is that it cannot be copied onto a blank, which means the property people are asking for is exactly the property that would destroy it.
So this is not a gap waiting for a clever application. It is the design working.
Frequently asked questions
Can I copy my car key onto a tag?
No. The credential is not readable, and a car that accepted a written copy would accept anybody's copy.
What about an older car with a plain key?
A mechanical key has nothing to copy electronically. NFC has no role in it at all.
Can a tag open a car door in an emergency?
No. A tag can carry the information someone needs at that moment, which is a different and more realistic use.
Why do phones open some cars then?
Because the manufacturer issued that phone a credential through their own process. Nothing was written to a tag.