Hi @liliann74228655, you can view plugins in XD by selecting Plugins > Browse Plugins from the menu (that's how I can do on mac, I think it must be the same in the Windows version).
About UI kits, I suggest checking @HARSHIKA_VERMA's answer here:
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Hi All,
We are aware that Get Ui kits are currently routing you to this help article: Access UI kits, plugins and app integrations where you are unable to download the UI Kits as the downloadable file links are not available on the page.
Please check this page for now: What Is a UI Kit? Examples & How to Use | Adobe XD Ideas, which might help you with the famous UI kits.
The team is working on adding the kits to the page and we will share the updates with you as soon as it's done.
We are sorry for the inconvenience.
Thanks,
Harshika
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You can also check https://www.xdguru.com/free-xd-ui-kits/ , hope this helps!