To use components from other packages like Fumadocs UI, pass them to your <MDXContent /> component.
You can also import them in MDX Files, but it is not recommended.
Deep Dive: Why?
Content Collections uses mdx-bundler to bundle MDX files.
To support importing a package from node modules, Fumadocs added a default value to the cwd option of MDX Bundler.
It works good, but we still do not recommend to import components in MDX files.
Reasons:
It requires esbuild to bundle these components, while it should be done by the Next.js bundler (for features of Server Components)
You can refactor the import path of components without changing your MDX files.
With Remote Sources, it doesn't make sense to add an import in MDX files.