What it writes
The HTML for the page's <head> is written as you type: the <title>, the meta description, a canonical link, Open Graph tags for Facebook, LinkedIn, Slack and iMessage, and a Twitter card tag. Leave a box empty and its tags are left out rather than written empty. Quotes, ampersands and angle brackets are escaped, so a title like Salt & Pepper cannot break the markup.
Two previews
Above the HTML are a search result, with meters for the title and description measured in pixels, and the card most apps show when the link is shared. The image is never loaded: the card shows where it would go and which site it comes from, and says so when the image address is not a full web address that other sites can fetch.
Questions
Does this fetch my page or image?
No. Nothing here contacts any other site: the previews are drawn from what you type, and the image box shows the image's address rather than the picture. Everything stays on your device.
Does X (Twitter) need its own tags?
Not many. X reads the og: title, description and image, so the only tag it needs of its own is twitter:card, which is summary_large_image when there is an image and summary when there is not.
Why is there no meta keywords tag?
Google has said since 2009 that it does not use it for ranking, so it is left out.