linktree - just a list of links, I guess you can choose a background and icon
carrd - pretty much lets you build a miniwebsite, you can add pictures, links, buttons, separate stuff into pages
and both have free, limited options, with linktree it seems like they keep more customization behind the paywall and analitics, with carrd, you're limited to 3 sites per account (nothing stops you from using another email if you need more...) and have some more advanced options locked (but those require some coding so if you're going for them, you're better off just making your own proper website :P)
personally I prefer carrd, because I can separate stuff into categories and not just have a one long list