Hi anonymous user,
The screenshot below shows several subsites are displayed on a classic page using content search web part. Per my understanding, are you meaning to display those subsites on a modern page using Highlighted content web part?
If yes, please take a reference to the following steps:
- Add Highlighted content web part on a modern page.
- Click "Edit web part".
- Select "Custom query" option.
- Select "All sites" in Source.
- Put query script in Query text textbox and click "Apply".
- Enable audience targeting.
- For other customization, please refer to this aricle: Use the Highlighted content web part.
After completing this, Subsites can be displayed on a modern page like what it is on a classic page.
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