When should a how title end in a question mark? [duplicate]

Your first scenario, the whole "How to build a smart car" should be treated as a noun phrase.

If you want to add a question mark, it is more common to expect a full question, including the Wh- word or adverb. You'd need: how (in this case), an auxiliary verb to form the interrogative, subject, main verb, complements and finally the question mark.

How do you build a smart car?

You can have question marks without a full interrogative scheme, but in my humble opinion that occurs when using the interrogative with surprise or exclamation intention, hence preferring the exclamation point instead.

How come you didn't pass the exam!