Tying together creation of multiple content types in a form
The Field Collection module is a quick and easy solution. It lets you nest entities within your content type as a field. So you could have a "Garage" Content Type which then has "Car" field of type (field collection) with "Number of values" set to unlimited.

At /admin/structure/field-collections you can add fields to this Car type.

On the Garage edit form you can add cars to your garage as shown below:



Then while viewing a Garage node, a user with sufficient permissions will see edit and add links which work just like you would expect. See below.



The downside with this approach is that since the Car's information is tied closely to a specific Garage, it's not easy to move a car from one garage to another or have one car in several garages, etc. Those use cases are possible, but you will have to write you own code however.