What if somebody put an AirTag in my vehicle.. could they find my car too. And how could I detect AirTags in my vehicle

Yes they could track you but Apple has set up several ways you would be notified of the presence of an Airtag that was not accompanied by its owner. Read the document in the link below:

What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support

Someone could place an AirTag in your car but you will find it. Because of when an AirTag is long separated from its owner, it will start to make noise that you will hear. Also, you will get a notification on your iPhone stating that an unknown AirTag is traveling with you.

Apple has implemented this measures to prevent that you can be tracked with an AirTag