If you won't configure anything then search engines and social media services will be scanning the source code of your website and picking random information that they think may be correct for displaying on their pages. It's sometimes good but more often it's not.
As theme creators we deliver you semantically correct HTML structure which is up to the newest standards and which search engines and social media services will see and parse correctly (but that doesn't mean they will extract the correct title, description and thumbnail).
To have them extract the correct title, description and thumbnail it's necessary to add meta tags to the source code of your website to inform them what to extract. The theme won't add any search engine or social media meta tags. Adding any meta tags for search engines and social media services is optional and is out of the scope of our support but we provide you the below information for convenience. You should consult third party resources to find out more about this subject.
Follow the steps to make your website appear nicely in search engines and social media services:
No. Such meta tags are a "functionality" layer and not a "presentational" layer. When you switch themes you'd expect all your meta tags to remain intact — they are unique for each individual post and page and hence they should be configured only with plugins.
Yes. You can investigate the source code of a few major websites and you'll see that many of them manually added meta tags with data that search engines and social media services later fetch. Such data typically includes (among others):
Yes. Some SEO plugins will offer you an option to fill your meta tags automatically with some data of your choice (for example the og:title meta tag may be filled with the post's title by such plugin). These plugins are designed to satisfy most cases but you can also create your own plugin that adds these tags to wp_head() based on your own criteria.