Hubert Schölnast has nicely explained the use of demonstrative pronouns.
However, I do not think that the phrase "Dieses ist ein Auto" is wrong. Yes, it is perhaps an unusul formulation and in most cases "Das ist ein Auto" should be preferred, but in a certain context I would nevertheless use it.
For example, on a parking lot I could point to a car and say "Dieses ist mein Auto". Of course, "Das ist mein Auto" would also do and most people would use this phrase. But "dieser / diese / dieses" has more emphasis than "der / die / das" and therefore it can be used correctly if one wants to identify a certain object in a collection of things or just to put special emphasis to something.
An example from the bible is Matthäus 3.17:
Dieser ist mein lieber Sohn, an dem ich Wohlgefallen habe. [Elberfelder Bibel]
But I have to admit that there other translations like
Das ist mein geliebter Sohn, an dem ich Wohlgefallen habe. [Zürcher Bibel]
Dies ist mein lieber Sohn, an dem ich Wohlgefallen habe. [Lutherbibel 2017]
See here.