What will 𝕏 show for this link?

Paste a link. This follows every redirect the way X's crawler does, reads the card tags off the page it actually lands on, and checks the image and robots.txt along the way.

How X reads a link

It follows HTTP redirects, and only those.
301, 302, 307 and 308 are followed to the end. A <meta http-equiv="refresh"> or a JavaScript location = is not — the crawler runs no JavaScript, so a page that redirects that way, or builds its tags client-side, has no card.
The tags, and their fallbacks.
twitter:card picks the layout. twitter:title falls back to og:title, twitter:description to og:description, twitter:image to og:image. Without a twitter:card tag you get the small summary card. The page's <title> and meta description are never used.
The image has rules.
JPG, PNG, WEBP or GIF, under 5 MB, no bigger than 4096×4096. A large card wants at least 300×157 and crops to about 2:1 — 1200×600 is the safe size. A summary card wants at least 144×144 and shows a square. The URL must be absolute and fetchable as Twitterbot/1.0.
robots.txt applies — to the image too.
Twitterbot honours robots.txt. A Disallow covering the page hides the card; one covering the image host (a CDN, often) leaves the card with a grey box. This is the most common cause of a blank card and the hardest to spot by eye.
X caches cards for about a week.
Fixing the tags won't change a card X has already seen. There's no official refresh any more; the usual workaround is to share the link with a query string it hasn't seen (?v=2).