This profile preview features the first two lines of the user's bio and the first two fields (commonly used for information like pronouns or verified links) which should hopefully make people more informed about who they talk to before they do so (similar idea as the reply prompts we launched a while back on Android).
As for fediverse accounts underneath link previews, for now this feature requires manual review from our side so that we can avoid the risk of malicious sites framing users as their authors, but we intend to build out a self-serve system to manage which sites you can be featured from very soon.